MECHANICAL OPTICS 
SLADE MECHANICAL 
OPTICS 
A Revision of the Book Formerly Called 
The Making of a Mechanical Optician 
Originally Written by the Late 
W. W. SLADE 
PUBLISHED BY 
The Keystone Publishing Company 
Philadelphia, U. S. A. Copyright 
1924 J 
The Keystone Publishing Co. CONTENTS 
Chapter I 
Selecting, Installing and Operating the Machinery 7 
Chapter II 
Care of Machinery 18 
Chapter III 
Lenses-Description, Marking and Cutting 25 
Chapter IV 
Marking Prisms 38 
Chapter V 
Cutting Complicated Lenses 65 
Chapter VI 
Edging 75 
Chapter VII 
Drilling 89 
Chapter VIII 
Mounting 98 
Chapter IX 
Bifocals 121 
Chapter X 
Frame Bending and Truing 142 
Chapter XI 
Surface Grinding 150 
Chapter XII 
Soldering and Repairing 184 
Chapter XIII 
Stretching, Shaping and Repairing Zylonite and Shell Frames .219 FOREWORD 
"Z "y' N the preparation of this treatise, the compilers have had 
in mind the interests of three classes of readers-the 
optometrist who wishes to operate his own shop, the 
optician whose specialty is mechanical optics, and the 
student of Optometry who must needs acquire a thorough under- 
standing of mechanical optics even though he has no intention 
of operating a shop in conjunction with refraction work. 
Treatment of the subject therefore begins with a description of 
the equipment needed to properly fit up an optical shop and 
touches next upon the care of the machinery. Then follows in 
logical order the work done in an optical shop, each subdivision 
of mechanical optical work being given a separate chapter. 
The present edition is the revision of a work originally written 
by W. W. Slade (now deceased), who in his treatise, which was 
entitled The Making of a Mechanical Optician, had laid a solid 
foundation of enduring usefulness. 
On this foundation several collaborators have helped to rebuild 
the book in the light of late developments in the field of mechan- 
ical optics. In this connection the publishers wish to acknowledge 
their indebtedness to and express their appreciation of the co- 
operation of W. E. Munday, F. H. Clements, L. L. Bugbee and 
Edwin H. Manwiller. 
We are also indebted to the following concerns for the Ioan of 
photographs or cuts, for permission to reprint from shop manuals 
and for suggestions on revision: American Optical Co., Bausch 
& Tomb Optical Co., B. B. W. Mfg. Co., General Optical Co., 
Geneva Optical Co., Globe Optical Co., Kryptok Co., Inc., One- 
Piece Bifocal Lens Co., Standard Optical Co., H. J. Stead Co., 
Stiles & Co. 
The book, which was so well planned in the beginning and so 
carefully revised at this time, we are sure will prove a potent 
factor in increasing proficiency in the field of Mechanical Optics. 
The Publishers. 
Philadelphia, Pa., 
December, 1923. CHAPTER I 
Selecting, Installing and Operating 
the Machinery 
A LTHOUGH every reader of this book will not, perhaps, be 
/"A called upon to equip an optical shop, it is nevertheless 
desirable that every student of mechanical optics be 
familiar with the necessary equipment as well as with the 
operations to be performed with such equipment. 
In this opening chapter, therefore, the necessary machinery is 
described and information given for guidance in selecting and 
operating the various units of an optical shop equipment. 
To an optician, whose sole interest is in dispensing, the equip- 
ment of a shop is a prime necessity, but to the optometrist, whose 
chief interest is in refraction, the operation of a shop (or labo- 
ratory) is a matter of choice to be determined solely by individual 
preferences or circumstances. 
To the student of Optometry this subject is of vital importance 
because later on, when he commences to practice, the effective- 
ness of his refraction work will depend upon the accuracy of the 
mechanical or laboratory processes followed in filling his prescrip- 
tions. His reputation as a refractionist will be materially affected 
by the way his orders are executed by the mechanical optician. 
To know how to order, what to expect and how to check up on 
fulfillment of prescriptions, he must necessarily understand the 
processes involved to an extent that he could perform any of them 
proficiently if necessary. 
Whether or not, as a graduate in Optometry, you decide to 
have your own shop or laboratory will doubtless depend upon 
whether in your opinion the advantages outweigh the dis- 
advantages. 
The advantages of a private shop, for example, may be summed 
up under the heads of quick service to your patients, extra profit 
on each transaction, appeal to prospective patients in the idea of 
your doing your own 3 work (if advertised) and the advantage 
of undivided responsibility. 
On the other hand you will of course ask yourself whether these 
advantages outweigh the trouble and annoyance of the increased 
7 MECHANICAL OPTICS 
8 
burden of business responsibility in supervising a shop as well as 
an office practice; the extra financial risk of mechanical invest- 
ment and upkeep; the additional overhead to be covered because 
of employes who must be kept on the payroll whether business is 
brisk or slack; the question as to whether promptness, accuracy 
and economy are not met by sending your work to a nearby 
prescription house, and whether the time required in keeping tab 
on a shop could not be more profitably employed in the refracting 
room. 
Whatever your decisions on these points, a knowledge of required 
machinery, how to install and operate it, is of vital value to you 
whether you are an optometrist, an optician or an optometric 
student. 
Figtire 1 
Selection of an optical shop outfit has been greatly simplified 
in recent years for today it is possible to purchase an entire out- 
fit, mounted on a specially designed bench as illustrated in Figure 
1, which shows a comprehensive outfit including composition 
hand edger, drilling machine, lens cutter, polishing and buffing MECHANICAL OPTICS 
9 
head and necessary pulleys, countershafts, etc. The bench is 5 
feet long by 2 feet wide and stands 2 feet 9 inches high. 
In these days of high real estate values, the amount of space 
needed is quite important. Hence the compactness of such an 
outfit as shown in Figure 1 is worthy of serious consideration. 
On even a narrower bench the work can be laid out and handled 
conveniently. 
In selecting individual units or purchasing a complete outfit as 
shown, there are many practical considerations involved which 
will be recited in detail in describing the several units that go to 
make up an optical shop outfit. 
Figure 2 
It certainly is false economy to buy cheap machinery. It gives 
constant trouble and wears out sooner, thus involving double 
investment. With the best equipment a great deal of the work 
is done automatically, requiring the minimum of attention and 
time and assuring accurate results even in the hands of a com- 
paratively inexperienced operator. 10 
MECHANICAL OPTICS 
After deciding on the bench, the next step is to select the frame- 
less machine. 
Among the most prominent on the market are the Gem, Junior, 
Superior, Wellsworth and Simplex. The Gem is a medium-sized 
machine, designed more especially for prescription work, but 
Figure 3 
stock can be ground on it also, though not in as large quantities. 
It has a composition stone, 18" diameter by 2" face, and it has 
been very popular with the shop men for several reasons, prin- 
cipally its simplicity, which permits its being taken apart and 
put together very easily. 
The Junior is a small-sized machine designed for heavy work 
in any shop. While it is equipped with a small stone, 12" diameter 
with face, it has large, heavy lens carriage, equipped with 
ball-bearings and renewable bronze bushings for all wearing parts. 
This greatly facilitates the replacement of worn parts and makes 
practically a perpetual machine. MECHANICAL OPTICS 
11 
The Superior is one of the latest additions to the several styles 
of frameless machines on the market. It combines all the features 
of the Gem and Junior machines, with the added feature that the 
oscillating mechanism runs in oil and requires no special atten- 
tion. It is equipped with a large stone, 18" diameter by 2" face. 
Although here shown complete on a pedestal this machine can 
be fitted on a bench equally as well. 
Figure 4 
The Wellsworth 84 is another excellent machine and durably 
built. The attachments are simple and are accessible and easy of 
adjustment. It is driven by worm gears, so designed that any 
oscillation can be secured. 
The Simplex supplies the prescription optician with a machine 
that will output conveniently in a limited space and grind from 
one to three lenses at a time. The machine can be set on any 12 
MECHANICAL OPTICS 
work table or on an iron stand. It is equipped with a carborun- 
dum stone, 10" diameter by face. 
Whichever machine you decide on be sure to get a composition 
stone, as a craigleith requires constant care and never works as 
satisfactorily. These composition stones are very hard, and will 
keep in shape sometimes for years. Either size is all right for 
hand w'ork. It might be said that it is really better to have a 
separate stone for hand work, as it permits keeping the frameless 
machine in better condition. While grinding over a machine the 
water runs off the hands on to the machine. This is full of grit, 
and unless you clean the machine every day it will soon become 
gummed up and in time refuse to work. If you install a machine 
with a small stone, however, you should have an extra stone, 
which can be obtained for about fifty dollars. The small machines 
are designed particularly for prescription w'ork. The stone is but 
12" diameter and 1" face. The machine, howrever, is built just 
as heavy as the large ones, and particular attention has been 
given to the wearing qualities. 
The idea of using this small stone is to take up less space and 
to require less power (a 1/6 h.p. motor will operate it). Also the 
stone can be replaced at small cost. Another advantage is that 
in case of repairs it can be boxed easily and shipped to the factory 
at small expense. This will be appreciated by anyone having 
had occasion to renew broken parts or turn up a stone on a large 
machine. Frequently it has been necessary to pay a man's 
expenses from the factory or wdiolesafg house to do this work, or 
if the machine has to be sent to the factory, it must be crated and 
shipped by freight. 
The selection of one of these machines will depend on the cir- 
cumstances and requirements of your wrork. The edge grinder 
should, however, be installed at the right-hand end of your bench, 
as you will find it more convenient. 
Drills 
There are many good drills on the market, and samples of these 
can be seen at all the large wholesalers, or a good idea of them 
can be obtained by consulting the different catalogs. You can 
obtain a low-priced diamond drill, without a centering device, 
that does excellent work, but requires more experience in center- MECHANICAL OPTICS 
13 
ing the lenses. There is a separate centering device on the 
market that can be obtained for a small sum. Opticians having 
old-style drills will find this a great convenience in marking 
lenses for drilling, as it centers them instantly and accurately. 
It also has a scale for drilling above and below center. 
If you can afford it, you should purchase a drill with the center- 
ing device and also an arrangement for drilling torics, such as the 
Universal (Standard Optical Company's) or the American Optical 
Company's drill. You should, by all means, use a diamond point, 
although these are not guaranteed. With a little instruction and 
by using care you will seldom break one. A turned diamond is 
best and these cost from ten to twenty dollars. 
Lens Cutters 
Next is the cutter, and this should be placed on a square board, 
so that it can be moved back out of the way when not in use. 
There are only a few on the market which are used to any great 
extent, and these are: Wellsworth, Little Gem, Standard, Uni- 
versal and Globe. The Wellsworth is arranged so that it cuts 
any size or shape by a simple turn of a knob. The Standard is 
operated by one large pattern with a groove in the under side. 
In place of a micrometer there is a five-sided barrel w'ith the two 
rolls that run in the groove in the pattern arranged at different 
distances, so that by turning it you can cut the shapes, having a 
difference between the length and width of ten, nine, eight, seven 
and six millimeters. This machine is furnished with a steel 
wheel, but should be obtained with a diamond, even at additional 
cost. 
The Little Gem has an adjustable pattern, and by adjusting 
both the pattern and micrometer head, according to a chart 
furnished with it, you can cut almost all ovals. While this 
scale is nearly right, we would suggest that you make a scale of 
your own, for the regular shapes, for accurate work. This can 
be done by cutting pld lenses and measuring them until you find 
just the right amount to allow for grinding. 
The Globe is arranged by a series of patterns, so that it will 
cut any size or shape. These are changed instantly by lifting the 
top gear. It has a micrometer, arranged with a pointer, so that 
the frameless sizes are read in millimeters, and the frame sizes in 14 
MECHANICAL OPTICS 
the regular way, 1, 0, 00, etc. This scale is also arranged so that 
it is set for regular and full eye, without any difficulty, and with- 
out consulting a chart. To cut an oval lens 40 x 33, for example, 
subtract the width from the length (this is seven millimeters). 
By placing No. 7 pattern in the machine and setting the pointer 
at 40 mm. you get the required size. 
Just here we will put in a word about diamond. Quite fre- 
quently we hear that someone wants a cutting diamond sharp- 
ened. This is impossible, as a cutting stone is a natural formation, 
and although it can be reset many times, it cannot be sharpened. 
In resetting, the stone is taken out of the mounting, and replaced 
at a different angle. These stones have from one to four cutting 
points, and after one has been reset two or three times, it is then 
necessary to get a new point. 
Drills are made of splints, which are obtained by cleaving large 
stones, or from natural stones, sharpened or lapped by hand. 
These can be resharpened a number of times, and should be kept 
sharp to do quick work. The bench should be equipped with a 
buffhead, having a taper screw and chuck, for small drills, burrs, 
polishing wheels, etc. As these are not furnished with the outfit, 
and they must be selected separately, the following are recom- 
mended: about six small twist drills, of different sizes, including 
the size for frameless glass screws and stud screws; two sizes of 
solder burrs, for burring out eye wire after soldering; two sizes 
of temple burrs for burring the joints of temples; a brush wheel; 
a felt wheel, about three or four inches' diameter will do; a cotton 
wheel, about six inches' diameter. This outfit will do all the work 
you are required to do. 
The Motor 
Next is the motor, and the first thing to do is to inquire of the 
power company what the current is. It probably will be either 
direct, 110 volts, or alternating, 110 volts, 60 cycles, and in asking 
for quotation, or ordering an outfit from the jobber, be sure and 
give this information, as it makes about twelve or fifteen dollars' 
difference in price. All quotations are given with direct current, 
and the alternating costs extra. 
If you have a direct current, a % h.p. shunt-wound motor 
should be used. This maintains its speed without the load, while 
a series-wound will speed up and the load must be kept on to hold MECHANICAL OPTICS 
15 
it down. You should see that you get a quiet one, as a magnetic 
hum is very disagreeable. There is a rheostat, or starting box, 
furnished with it, and if the outfit is bought complete it will be 
wired up on the bench with a switch and cut-out. It is only 
necessary then for the electrician to connect the wires, and you 
are ready to staid. 
If you have alternating current, we should recommend an 
automatic starting, % h.p. This requires no starting box, which 
is quite large for this current, and is also started quicker. In 
buying a motor, the speed is an important matter, and many of 
the shops that have been fitted up by opticians themselves are 
equipped with high-speed motors. These have been installed 
either from inexperience or on account of the low price. A 
slow-speed motor runs with less noise and there is less slipping 
of the belts. Then again you do not require such large pulleys. 
In belting up the motor, an endless belt should be used, so there 
will be no pounding of the lacing and no vibration. This is very 
important, if you want a smooth-running outfit. 
From the foregoing general outline of the outfit, you will see 
that there is a great advantage in purchasing this complete and 
benefiting by the experience of the optical machinery manufac- 
turers. They have made a study of shop troubles, and have 
succeeded in eliminating many of them. Occasionally you will 
find an optician who has assembled his outfit from various sources 
with the idea of saving money, but in the end he finds it has cost 
more than any first-class outfit on the market. 
Tools Needed 
Now in regard to tools and supplies. These, of course, are not 
furnished. You will need the following: A pair of breaking tongs, 
cribbers, a hand-diamond, a set of the brass patterns for cutting 
odd lenses, round-nose pliers, snipe-nose pliers, optician's hollow 
chop pliers; flat-nose pliers, strap pliers, angling pliers, cutting 
pliers (and as there are plenty of special pliers on the market it is 
advisable to look them over and get a few of the best ones); an 
assortment of files, including rat-tail; a millimeter rule; broaches, 
drilling fluid, bifocal cement and polishing material. This assort- 
ment covers the essentials and can be added to as occasion 
requires. 16 
MECHANICAL OPTICS 
Installing the Outfit 
When the bench outfit comes to you, it will be crated, and after 
it is opened up, set in place and connected with the current, it 
should be wiped up to take off the grease which is put on to keep 
it from rusting in transit. Then look at the bearings, especially 
the boxes on the stone and main shaft. These may have been set 
up tight in shipping. If so, the screws should be loosened a little 
until they turn freely. 
Everything should then be well oiled and then the motor can 
be started. It should be run a few hours, feeling the bearings 
occasionally to see that they do not warm up. If so, they should 
be loosened up a little more. Possibly the belts may be a little 
loose, and even if they are all right at first they will have to be 
shortened later, as all new belts will stretch. If the motor is a 
good one it will have an adjustable base, so that it can be adjusted 
to take up the slack. After taking up all you can in this way have 
a piece taken out at some belt manufacturer's, and do not attempt 
to lace it. All other flat belts will be laced or fastened with some 
special motor clip, however, and these you can take up yourself, 
but always use light, thin lacings to make it run as smoothly as 
possible. The round belts will probably be put together with 
hooks, and they will require shortening very often. In shortening 
belts do not get them too tight; it is better to run them a little 
loose, and if they slip it is probably because they are too hard and 
dry. Applying a good belt dressing or a little castor oil occa- 
sionally on the inside while running will make belts soft and 
pliable. Never use resin or anything of that nature. 
After everything is running smoothly, you can then put water 
on the stone and place some thick lenses in the machine and allow 
them to grind a while. In starting a new composition stone, it 
will absorb the water very quickly, and for the first hour or so 
watch it carefully and see that it does not run dry, as this will 
rough it up and may require turning. The stone can be kept wet 
by a drip, either from a tank, or it can be connected with the 
water supply, but you should then arrange for the outlet, or it is 
liable to overflow. Some machines have a sponge pan on the back, 
filled with water, the sponge acting as a wick, keeping just enough 
water on the stone. This is recommended as it is a much cleaner 
method. Often a stone shipped from the factory is not quite MECHANICAL OPTICS 
17 
ready to grind on, but by running thick lenses in the machine for 
a while it will smooth up very quickly. If it does not, a piece of 
the same material, held by the hand, will polish it. 
All the frameless machines are equipped with truing devices 
and it is well to true the stone with a turning diamond every little 
while to keep it in perfect condition, rather than allow it to get 
badly out, as it is then quite a job to get it back. All that is 
necessary to do is to set the truing device up until the diamond 
just touches the grindstone and allow it to run a few minutes until 
the surface of stone is flat and true on circle. As the stone 
revolves it trues itself automatically. The diamond is placed in 
the truing device and operated the same as the carborundum 
block. After you have turned the stone true, it will be very 
rough; then by replacing the carborundum you can smooth it, the 
same as before described. 
Use of the drill and cutter will be described later, but they can 
be tried, and if they do not suit have them exchanged at once, as 
they are not guaranteed. Do not spoil them and then claim they 
were never right. If you do not get good results in cutting, it is 
probably because you have not used just the right pressure. Start 
lightly, and on each lens gradually increase the pressure until you 
get a good, clean cut. 
The cost of operating is trifling, as a X h.p. motor uses very 
little current, costing not over one dollar a month. 
The illustrations shown on the preceding pages will give an idea 
of the arrangement of benches widely used. CHAPTER II 
MANY opticians, especially those having the new style 
benches, are very particular to keep them clean, and, 
although their intentions are good, this is often the cause 
of trouble. A machine that is running constantly in water needs 
considerable oil, and although a machine covered with grease does 
not look very neat, it is really the most practical. If the machine 
is not wiped carefully the water and grit are deposited on the 
metal, causing it to rust or bind very quickly. 
The proper method for the care of a frameless machine is to 
wipe off carefully the water and grindstone grit, but see that it is 
pretty well covered with oil. All machines are supplied with oil 
cups, and these, if kept well filled, will take care of the bearings 
to a certain extent, but all bright parts exposed should be well 
oiled also. This includes the gears, shafts and all working parts. 
The grindstone grit, which is a mixture of glass and stone, is partly 
the cause of trouble and allowing the bearings or shafts to run 
dry is usually responsible for the rest. 
A machine that has the best of care will, after a time, run hard, 
and as soon as this is noticed it should be taken apart and the 
trouble located. The first symptom is that a great deal of spring 
tension is needed to hold the lenses against the stone. This may 
be noticed on account of the lenses not grinding very quickly, or 
that the lenses have a tendency to pull away from the stone. If, 
for any reason, this may have escaped the operator's attention, 
the machine will gradually get worse, and suddenly it will pull 
back and forth from the stone. This will cause the lenses to slip, 
and possibly to chip or break. This motion is caused by friction 
and, by studying the transmission a moment, it is easily under- 
stood. The shaft or belt, whichever way the machine may be 
driven, is arranged so the lower shaft turns in the opposite direc- 
tion to the stone, and the small gears on this shaft mesh with the 
larger ones on the upper shafts. The upper shafts must neces- 
sarily revolve in the opposite direction from the lower shafts, or 
Care of Machinery 
18 MECHANICAL OPTICS 
19 
toward the stone. If there is friction anywhere on the lower shaft 
it has a tendency to cause the upper shafts to revolve in the same 
direction as the lower, and, consequently, it turns as far as the 
spring will allow it, and then jumps backward. 
The first step is to locate which shaft is the cause of the trouble. 
By removing the small gear on the left side of the lower shaft, it 
will leave the upper one on the left free. As this particular one 
has either a ball bearing or a cone bearing, there may be trouble 
at this point. If the difficulty is located in this shaft, the re- 
mainder of the machine can be turned with the hand wheel. If 
it turns freely, it will not be necessary to take any more of the 
machine apart, unless, possibly, you may intend to give the 
machine a thorough overhauling. The shaft that is the cause of 
the trouble must be removed and thoroughly cleaned. If it is 
simply gummed up with grease, this can be removed with kero- 
sene, but if the shaft has been running dry, and this is the most 
common trouble, it is probably scored so that it binds. A fine 
grade of emery cloth should be used to smooth it, rubbing it down 
until it fits easily. Before it is replaced the bearing should be 
cleaned thoroughly with kerosene and the oil hole or cup cleaned. 
A machine may be oiled regularly, but oftentimes the oil be- 
comes gummed, stopping up the oil cup, and yet escapes the 
attention of the operator. If it is the lower shaft that is causing 
this trouble, there is nothing more to do to it, but should it be 
either of the upper shafts, the ball or cone bearings must be looked 
over carefully. If any of the balls are broken, replace them as 
soon as possible, although the machine can be run temporarily 
with one or two missing. If it is a cone bearing, it may have 
become worn or scored a little and may need to be turned. If 
the machine is one that has a screw-clamping arrangement for 
holding the lenses, this will have to be taken apart and looked 
over carefully; it may be only gummed up or a spring may be 
broken, which will have to be replaced. 
Keeping the Bearings Perfect 
A machine that has been running one year will show little or 
no wear, but if it has been running for a longer period, the bear- 
ings may be considerably worn. In this case they should be fitted 
with new shafts and bushings, and if one is to have accurate work 
there can be no great amount of play in the bearings. It will 20 
MECHANICAL OPTICS 
depend, however, on the machine; if it is so constructed that the 
pattern is near the lenses, the wear does not make as much differ- 
ence as if it were placed on the other end of the shaft. If a 
machine needs repairing, the front part can be removed and 
shipped to the factory, by express. It is unnecessary to ship the 
stone, stand, etc., unless one is not mechanically inclined. When 
it comes to a question of repairs, it is cheaper and better to send 
it to the factory where it is manufactured, as they have the parts 
in stock and understand what is required better than some local 
machinist. 
After the machine is in running order again, there are some 
points which should be looked after to put the machine in first- 
class condition. The bearings of the stone will probably show 
signs of wear and, as all the best machines are constructed with 
split boxes, all that is necessary to do is to tighten the four set 
screws occasionally, to take up the play (Figure 5). A bearing 
Figiire 5 
of this kind is babbitted-that is, it is lined with a composition 
of soft metal, consisting of lead, tin and antimony. This forms 
one of the most satisfactory bearings, although not the best nor 
most expensive. The theory is that one soft metal and one hard 
metal, running together, cause the least friction and the least 
wear. When a bearing of this description is made, the soft metal 
is poured, when hot, into the cast-iron box around the shaft, so 
that it is shaped to it perfectly. It is, however, left a little large, 
to allow for a "take up'' as the bearing wears. After the wear 
has been taken up as much as possible the boxes must be rebab- 
bitted; this can be done by any local machinist. To obtain the 
best results in lens-grinding, these bearings should be kept in MECHANICAL OPTICS 
21 
shape, although they can be run for years with loose boxes with- 
out much serious trouble. It is when the stone is to be turned 
that loose bearings cause the greatest trouble, as it is impossible 
to turn it true if there is any "play" in the boxes. 
The stone at this time may need turning, possibly on account 
Figure 6 
of ridges near the edges, caused by the lenses not covering the 
entire surface of the stone. The truing device is supposed to take 
care of this wear, but this device is usually neglected by most 
opticians. The correct way is to set this up to the stone every 
day for a few minutes, to offset the amount of wear caused by 
the grinding. If it is used only occasionally, it requires quite a 
little time to put the stone in first-class shape with this device; 
consequently the most satisfactory way is to turn the surface off 
squarely with a diamond and. then finish it with a truing device. 
This operation was described earlier in this chapter. 
If the surface of the stone shows no signs of wear it still may 
need turning,on account of its being out of round. This will make 
no great difference in hand grinding, but in machine work it will 
cause ridges or facets to appear on the edges of the lenses (Figure 
7). When this defect is found, many grinders look for trouble 
in some part of the machine, but this is always caused by the 
stone's being out of round. As stated before, it is impossible to 
turn it true unless the bearings are in perfect condition. 22 
MECHANICAL OPTICS 
After a stone has been turned a number of times the gage con- 
trolling the size of lenses will need adjusting, to allow for the 
reducing in size of the stone. Every machine is constructed so 
that this adjustment is very simple. All that is neces- 
sary is to grind a lens with the gage, set at any size, and, 
when it is finished, measure it accurately with a rule. 
The gage or pointer should then be loosened and set to 
this size. By consulting the directions which accompany 
each machine, anyone can easily set his machine without 
difficulty. 
The contact block, on which the pattern or former 
rides, also may show signs of wear. This, as it becomes 
grooved, will change the shape of the lenses, although 
they will not be exactly like the patterns used. This 
makes no particular difference when grinding pairs, but 
when matching an old lens they will not be alike. On 
some machines it will be necessary to fit a new plate, and 
on others the block can be reversed. After both sides are 
worn a new one then should be fitted. 
Figure 
7 
Care of the Drill 
With the usual amount of work in a small shop, the drill should 
require very little repairing. The upright spindle will wear for 
years, but possibly the idler shaft and idlers may have become 
worn so they rattle. This does no particular harm, except that 
the noise is objectionable. Any local machine shop can bush 
these bearings at a slight expense and it is well worth the trouble 
and expense to eliminate the noise. The centering device may 
require some attention, as this is operated in grit most of the time. 
With only a slight amount of wear, this can be taken up in a 
machine shop or at the factory, but when it gets very loose it is 
better to exchange the whole machine for a new one, paying, of 
course, the difference in cost. Any good drill should wear for 
years, however, before this part will cause trouble. After drilling 
for six months dr a year, the diamond should be repointed. Many 
are using slow-cutting drills, in ignorance of the fact that they 
can be sharpened at a slight expense, and these drills not only 
exhaust the patience, but also may break lenses, on account of 
the unnecessary pressure required. 
The cutter should require no repairing, except possibly the MECHANICAL OPTICS 
23 
diamond may need resetting. Do not confuse a diamond, used 
for cutting, with a drill. A drill can be sharpened, but a cutting 
diamond must have a natural point. In resetting, the stone is 
Figttre 8a 
Figure 8b 
taken out and reset at a different angle so that a new point can 
be used. When it is necessary to have your diamond reset, send 
it to your wholesaler, so that it may be done by one familiar with 
the requirements of a machine for optical work. A regular 
diamond-setter may not have had experience in the optical line, 
consequently the best results are not obtained. 
The buff head will probably need to be taken up in the bear- 
ings. This is not necessary, except that it may rattle so the noise 
is objectionable. This machine is also constructed with split 
bearings, so they can be taken up. 24 
MECHANICAL OPTICS 
The Motor 
The motor should require no attention for some time. After 
a while the carbon brushes will wear down, so that it will be neces- 
sary to replace them. These cost but very little and can be 
obtained from any motor dealer. After some years the part of 
the armature called the "commutator" will, perhaps, become 
worn from contact with the brushes. When this is worn to any 
great extent, it should be sent to a motor repairer, who can turn 
this off in a lathe. The ordinary care of a motor requires regular 
oiling and occasionally, in the event of sparking, the commutator 
can be cleaned or smoothed with a piece of 00 sandpaper. 
The bearings on the main shaft, and also any countershafts, 
should be looked over and any wear taken up by tightening the 
set screws. If any belts are slack, they should be tightened. 
Small flat belts, or those driving the stones, can be laced. There 
are several methods of connecting but rawhide lacings are the 
most satisfactory as they lie flat and are prac- 
tically noiseless. Hook and metal connections 
make a click, which is objectionable. 
Lacing the Belt 
In lacing a 2-inch flat belt, three small holes 
should be punched on each end, then two more in 
a line at a reasonable distance back of them. 
(Figure 8a.) On a 1-inch belt, 2 holes in each end 
are sufficient. (Figure 8b.) The lacing should be 
thin and passed through the holes as shown in 
the illustration. 
Round belts can be connected with steel belt 
hooks. (Figures 9 and 10.) When flat belts slip, 
pour a good belt dressing or a little castor oil on 
the inside, while running. Never use any other 
substance. 
Make it a point to keep your machinery in 
good running order. To have a noisy shop or outfit shows lack of 
attention to the small details. 
Figure 
9 
Figure 
10 CHAPTER HI 
Lenses-Description, Marking and Cutting 
THE following definitions provide an essential foundation 
for an understanding of the subject of this chapter: 
1. Lens: A lens is a piece of glass or other transparent 
substance bounded by two refractive surfaces. 
2. Siibstances from which ophthalmic lenses are made: 
a. Crown (commonly used). 
b. Flint (segments of Kryptok bifocals). 
c. Rock Crystal (pebbles). 
3. Index of Refraction: The relative density of a transparent 
substance or the comparative time required for light to travel a 
definite distance in different substances. Indices of refraction of 
various substances are as follows: 
Air 1.000294 
Water 1.3336 
Crown glass 1.54 
Flint glass 1.635 
Ordinary sheet 1.53 
Canada Balsam 1.53 
Diamond 2.487 
Alcohol 1.365 
Rock Crystal 1.548 
4. Refractive Surfaces are classified as follows: 
a. Plano. 
b. Spherical. 
c. Cylindrical. 
d. Torodial (toric). 
Plane Surface: A fiat or even surface; one without elevations 
or depressions. 
Spherical: A spherical surface is a section of the surface of a 
sphere (ball) or a portion of a surface which if continued in all 
directions would form a sphere. It is rounded to the same curve 
in all directions. It may be either convex or concave. 
25 26 
MECHANICAL OPTICS 
Cylindrical: Geometrically a cylinder is a solid bounded by a 
cylinder surface and two plane parallel surfaces. A cylinder 
surface is the curved face of a section of a cylinder. In shape it is 
similar to a section cut lengthwise from a piece of pipe, curved in 
one direction, around the pipe, and straight (plane) in the one 
direction, the long way of the pipe. It may be either convex or 
concave. 
Torodial Surface: The name of this surface is derived from the 
Latin architectural term torus meaning a large round moulding 
at the base of a column. It may be originated by making a globe 
or ball swing in a horizontal plane around an upright pole. A 
toric surface may be compared to an automobile tire which has 
a steep, sheer curve around the tire, about four inches, and a 
much flatter curve around the wheel, about thirty-four inches. A 
toric surface is one that is engendered by a circle which turns 
about an axis situated on the plane of the circle. 
5. Spherical Lenses (S. Sph.): Spherical lenses are so named 
because their curved surfaces are sections of a sphere. Some 
spherical lenses have one surface plane but may be in various 
forms such as: 
Plano convex Plano concave 
Double convex Double concave 
Periscopic convex Periscopic concave 
Meniscus convex Meniscus concave 
A spherical surface refracts rays of light equally in all meridians. 
6. Cylinder Lenses: Cylinder lenses are so named because their 
curved surfaces are sections of a cylinder, parallel to their axes. 
They are of two kinds: piano convex and piano concave. 
Axis of a Cylinder: That part of a cylindric lens which is 
parallel to the axis of the original cylinder of which it is a' part is 
spoken of as the axis of a cylinder. It is the meridian of no 
refraction. 
A convex cylinder converges parallel rays of light so that after 
refraction they are brought to a straight line. 
7. Sphero-Cylinder Lenses: A lens with one surface spherical 
and the other surface cylindric. A lens with two focal planes and 
two foci. MECHANICAL OPTICS 
27 
Kinds of Sphero-Cylinder Lenses 
Sphere Cylinder 
Convex Convex 
Convex Concave 
Concave Concave 
Concave Convex 
8. Cross Cylinder: A lens with both surfaces cylindrical. A 
lens with two focal planes and two foci. Kinds: 
Cylinder Cylinder 
Convex Convex 
Concave Concave 
Convex Concave 
The axes of cross cylinders are usually at right angles. It is 
not necessary that they be at right angles, although all cross 
cylinders are sphero-cylinders in power, and are therefore equal 
to some sphero-cylinder in form. 
9. Toric Lenses: A lens having one surface torodial, and the 
other surface spherical; or both surfaces may be torodial. Usually 
the convex surface is torodial and the concave surface is spherical. 
Base Curve: The base curve is the curve of least intensity or 
greatest radius on the torodial surface. The base curve of any 
system of stock lenses is the curve which characterizes the whole 
system because of its use as a curve of one surface of each lens 
of that system, regardless of the dioptric power of the lens as a 
whole. The base curve is expressed in diopters. It is the axis 
of that surface. 
10. Base Curves of Stock Lenses: 
Stock Name 
Plano convex and concave 
Double convex and concave 
Periscopic convex (Pcx) 
Periscopic concave (Pcc) 
Meniscus convex (Mcx) 
Meniscus concave (Mcc) 
Flat cylinders 
Flat sphero-cylinders 
Toric cylinders rough (finished 
on cyl. side only) 
Base Curve 
Plano 
A curve equal to one-half 
the total power 
- 1.25 D. 
+ 1.25 D. 
- 6.00 D. 
+ 6.00 D. 
Plano 
Plano 
+ 6.00 D., - 6.00 D. and 
+ 9.00 D. 28 
MECHANICAL OPTICS 
Toric cylinders convex finished 
(uncut) 
Toric cylinders concave finished 
(uncut) 
Toricp sphero-cylinders 4~ = 4~ 
(uncut) 
Toric sphero-cylinders - = 4~ 
(uncut) 
Flat prisms, piano, sphero, cylin- 
der, sphero-cylinder and rough 
Meniscus convex prisms 
Meniscus concave prisms 
Toric prisms, cylinder or sphero- 
cylinder 
+ 6.00 D. and + 9.00 D. 
+ 6.00 D. 
4- 6.00 D. and + 9.00 D. 
4- 6.00 D. 
Plano 
- 6.00 D. 
+ 6.00 D. 
+ 6.00 D., - 6.00 D. or 
4- 9.00 D. 
Kryptoks, fiat 
Kryptoks, meniscus 
Kryptoks, toric, cylinder and 
sphero-cylinder 
Ultex One-Piece, meniscus cylinder 
and sphero-cylinder 
Plano 
Same as meniscus 
- 6.00 D. and -3.00 D. 
From + 4.87 D. to 4~ 
8.00 D. inclusive. 
T TRANSPOSITIONS 
Transposition is changing algebraically the form of a lens, 
leaving the optical value the same. 
To simplify the many rules for transposing lenses, and to avoid 
the use of many signs, the following two rules, using the algebraic 
method of addition and subtraction, will solve all our problems, 
and enable us to keep the rules at our finger tips. 
In algebraic addition, when the signs of the sums are alike, we 
add using the same sign, and when the signs are unlike, the differ- 
ence is taken, using the sign of the greater sum. 
In algebraic subtraction, change the sign of the subtrahend 
(sum to be subtracted), and proceed the same as in addition. 
To determine the numerical value of a spherical lens, add the 
value of its surfaces. For instance: a spherical lens having a 
numerical valueof a + 2.50diopters on one surface and -1.25 I), 
on the opposite surface, the result will be T 1.25 sphere. Remem- 
ber that such values that represent like surfaces, such as two MECHANICAL OPTICS 
29 
spheric surfaces, or two cylindric surfaces (when the axes are the 
same), can be added or subtracted. 
Rule I: Piano-cylinder and sphero-cylinder. 
Add the numerical value of the sphere to that of the cylinder, 
algebraically, change the sign of the cylinder (+ to - or - to +) 
and reverse the axis 90 degrees. 
If the axis of the original cylinder is 90 degrees or less than 90, 
add 90. If the axis is more than 90 degrees subtract 90 degrees. 
Example I: + 2.50 sph. O - 1.00 cyl. ax. 90°. 
+ 2.50 sph. O - 1.00 cyl. ax. 90° 
- 1.00 
+ 1.50 sph. 0 + 1.00 cyl. ax. 180° 
Example II: + 1.50 sph. O - 2.50 cyl. ax. 15°. 
+ 1.50 sph. O - 2.50 cyl. ax. 15° 
- 2.50 
- 1.00 sph. O + 2.50 cyl. ax. 105° 
-Example III: - 1.00 sph. O - 2.00 cyl. ax. 45°. 
- 1.00 sph. O - 2.00 cyl. ax. 45° 
- 2.00 
- 3.00 sph. O + 2.00 cyl. ax. 135° 
Example IV: + 2.50 cyl. ax. 165°. 
0 sph. O + 2.50 cyl. ax. 165° 
+ 2.50 
+ 2.50 sph. O - 2.50 cyl. ax. 75° 
Rule II: Cross-cylinder (axes at right angles). 
(a) Use either cylinder for the new cylinder. 
(b) For the value and sign of the new cylinder, 
subtract algebraically the numerical value 
of the cylinder used for the new sphere, 
from the remaining cylinder. 
(c) Use the axis of the remaining cylinder. 30 
MECHANICAL OPTICS 
Example I: + 1.50 cyl. ax. 90° O + 2.50 cyl. ax. 180°. 
(a) + 1.50 sph. 
(b) + 1.50 cyl. ax. 90 O + 2.50 cyl. ax. 180° 
+ 1.50 
+ 1.00 cyl. ax. 180° 
(c) + 1.50 sph. O + 1.00 cyl. ax. 180° 
Example II: - 1.00 cyl. ax. 15° O + 2.00 cyl. ax. 105°. 
(a) - 1.00 sph. 
(b) - 1.00 cyl. ax. 15° O + 2.00 cyl. ax. 105° 
- 1.00 
+ 3.00 cyl. ax. 105° 
(c) - 1.00 sph. O + 3.00 cyl. ax. 105° 
Example III: - 4.00 cyl. ax. 10° O + 1.00 cyl. ax. 100° 
(a) + 1.00 sph. 
(b) - 4.00 cyl. ax. 10° 0 + 1.00 cyl. ax. 100° 
+ 1.00 
- 5.00 cyl. ax. 10° 
(c) + 1.00 sph. O - 5.00 cyl. ax. 10° 
Example IV: - 2.00 cyl. ax. 30° O - 3.50 cyl. ax. 120° 
(a) - 3.50 sph. 
(b) - 2.00 cyl. ax. 30° O - 3.50 cyl. ax. 120° 
- 3.50 
+ 1.50 cyl. ax. 30° 
(c) - 3.50 sph. O + 1.50 cyl. ax. 30° 
Selecting Stock 
In selecting the stock of lenses quite a little capital can be 
invested, or you can start small and increase it from day to day. 
A stock varies in price from $50 to $500, but the usual amount 
invested is from $200 to $300. This will include principally 
+ O + compounds, a few - O -, piano-cylinders, torics, 
Pcx, sphericals, regular and piano wafers. Practically all lenses 
come 47 mm. round, uncut. The + O - and - O +com- MECHANICAL OPTICS 
31 
pounds, compound wafers and rough cylinders (provided you are 
to do surface grinding) can be ordered daily, until you can afford 
to stock them. 
All uncut lenses are put up in single envelopes, with the axes 
dotted, and sphericals in the same way, but not dotted. If you 
have had no previous experience in ordering, a good plan is to 
see your wholesalers and tell them that you wish to place an order 
for a quantity of rough lenses, stating the amount you wish to 
invest, and let them submit lens sheets, showing the assortment 
and quantities that can be obtained for this figure. As they are 
constantly making them up, they can best advise you. You can 
then look over the list and change the assortment as you see fit. 
If you have made no provision for carrying your stock, you would 
do well to get an uncut lens cabinet. This is made of oak with 
galvanized iron drawers, with wood front and back, also all wood 
or all metal. 
A good way to keep your stock ordered up is to have a box 
handy, and as you use the compounds put the envelopes into it 
and order from them. 
In selecting the lenses for a prescription it will be necessary 
to have a pair of calipers or lens gage, graduated in .1 mm. With 
this the lenses are calipered in the center, allowing .4 mm. for 
each diopter. For example, a piano lens is the same thickness 
on the edge as in the center, and if you wish to make a pair of 
glasses two-strap thickness, having a piano in one eye and + 1 in 
the other, we select the piano first 2 mm. thickness and the ff- 1, 
2.4 in the center. If the lens was concave, it would be 1.6 mm. 
In this way the edges are made the same thickness. In cases 
where there is to be a compound in one eye and spherical in the 
other, it is well to look at the compound and use your judgment 
as to which would be best-a double convex or periscopic. If the 
compound is + O +, a Dex. would make a better match; whereas 
if it were + O -, a Pcx. would be nearer the curve. 
For marking you will need quite a stiff pen that does not 
scratch, a bottle of black waterproof ink and a protractor gradu- 
ated in five degrees. These are furnished by some wholesalers as 
advertisements, with a decentering scale on the back, which is 
very convenient. For cross lines use a card about the size of a 
reading test type, and draw heavy black lines at 90° and 180°. 32 
MECHANICAL OPTICS 
This will be used for medium and strong powers; for the weak 
powers you could draw the lines on a wall about twenty feet dis- 
tant. The best results can be obtained only by using one of the 
several good lens markers on the market. In marking, first 
Figure 11 
decide which surface will be next the eye and then always mark 
on this surface. The rule is: The greatest concavity, or the least 
convexity, next to the eye, always having the cylinders on the 
same side, except in extreme cases. We will first select a sphere, 
Figtire 12 
and all that is necessary in this case is to dot the center. We find 
the center by looking through the lens at crossed lines, and the 
lines will appear broken (Figure 11). MECHANICAL OPTICS 
33 
Then move the lens until the lines are continuous and place a 
dot where the lines cross (Figure 12). A cylinder usually has the 
axis running from corner to corner, and in lining it up the lines 
Figure 13 
will appear broken and perhaps twisted (Figure 13). By turning 
the lens they will line up so that they appear straight and by mov- 
Figure 14 
ing in a horizontal and vertical position they will be continuous. 
Then place two dots on the lens, one at the top and one at the 34 
MECHANICAL OPTICS 
bottom. Now lay it on the protractor with these dots at what- 
ever axis the lens is to be cut. Draw a line across the lens at axis 
180°. This will be the mechanical axis or cutting line, as it is 
Figure 15 
usually called. Remember that whatever axis is to be cut the 
cutting line is always drawn at 180°. For example, axis 45° will 
be placed as shown in Figure 14, axis 90° as in Figure 15. 
A compound is lined up in the same way as a cylinder, and 
the lines will look the same, but when you have it in position three 
dots must be placed on the lens, one at the top, one at the bottom 
and one in the center. It is then laid on the protractor the same 
way, but be sure that the center dot is on the center of the chart 
Figure 16 MECHANICAL OPTICS 
35 
or your lens will be decentered. If you wish to obtain some pris- 
matic power you can decenter it, using the following rule: A lens 
decentered ten millimeters will produce a prism power of as many 
diopters as the focus. Thus a -f- 1 D. lens, decentered 10 mm., 
will have a prism power of 1 ". By this you will see that a + 
Figure 17 
1 D. lens, decentered, 2.5 mm., would give us % prism. A mis- 
take is often made, however, in ordering a lens decentered 6 or 
8 mm. This cannot be done, as the stock lenses are not large 
enough. Extra large lenses could be used, but these would cost 
more than to have a pair surface ground. With the large sizes 
that are now being used a lens cannot be decentered over 2 mm. 
In marking a lens to be decentered, it is dotted in the usual way 
and then moved on the chart the amount necessary and a cross 
line drawn on the cutting line (Figure 16). 
Before the lenses are cut they should be neutralized to be sure 
the power is correct. If you depend on the lens measure it should 
Figure 18 
be kept accurate by frequent adjusting. With a lens measure 
you can determine the curve value of a surface, but it does not 
supply the power of a lens. 
Very little can be saved in buying torics uncut, as the risk of 
breaking is too great. However, if you prefer to grind these 
yourself they are handled just the same as flat lenses in marking. 36 
MECHANICAL OPTICS 
We will give a few suggestions for ordering, so that you will be 
sure to get the best results. If all prescriptions for torics were 
filled as written a great many would have no toric effect. 
You should, of course, understand the transposition of lenses 
as explained at the beginning of this chapter, and transpose them 
to the best form before sending in the prescription. If you do 
Figure 19 
not you should state that you want the prescription filled in the 
best form. In ordering plain cylinders it will make no difference 
which way they are ordered, unless wafers are to be fitted. If a 
+ cylinder was ordered, the lens would be ground as written and 
would have the cylinder on the outside, and a 6 D. curve on the 
inside (Figure 17). If a - cylinder was ordered it would be 
ground with the cylinder on the inside and a + 6 D. curve on the 
outside (Figure 18). If wafers were to be fitted, the cylinder 
'should be on the outside, so that the wafers could be cemented 
to the inside surface. It would then be necessary to transpose 
Figure 20 
to a compound. For example: A - .50 cylinder, axis 180°, would 
be - .50 sphere O + .50 cylinder, axis 90°. This lens would, of 
course, cost extra. 
The ordering of compounds is a more difficult matter and the 
optical workman should know just what surfaces are to be ground. 
For example: If a + 1 sphere O + .50 cylinder, axis 90°, was 
ordered, it would be ground as written, and the lens would be MECHANICAL OPTICS 
37 
+ 6 0 + 6.50 on the outside and a - 5 sphere on the inside 
(Figure 19). 
By this you will see that the toric effect has been reduced 1 D. 
While it does not matter much in this case, the toric effect is 
gradually reduced until with a combination of + 6 sphere O + 
.50 cylinder, axis 90°, it has a piano effect and this would be 
ground with + 60 + 6.50 on the outside and piano on the inside 
(Figure 20). 
All + and + combinations with a spherical stronger than + 3 
should be grotind on 9 base curve. For example: + 4 sphere O + 
Figure 21 
2 cylinder, axis 90°, if ground as written, would have only - 2D. 
inside curve. If it is transposed it would be ground with - 5 D. 
inside curve. This lens costs more, but will give better satisfac- 
tion to your customer. If you understand this before ordering 
you can explain it to your customer and charge more for your 
lens accordingly. Concave combinations are transposed in the 
same way. When wafers are to be fitted they should be trans- 
posed so that the cylinders are on the outside. 
In combinations where the spherical power exceeds + 3, the 
distance lenses shoidd be ordered on a + 9 curve. Thus a combi- 
nation + 3 sphere 0 + 1 cylinder would be + 9 O 10 on the 
outside and - 6 sphere on the inside (Figure 21). This gives you 
the regular - 6 toric effect, and although it increases the expense 
it makes a much better lens. CHAPTER IV 
Marking Prisms 
OPTICIANS, as a rule, prefer sending their prism work to 
the prescription houses. Some, however, purchase the 
lenses uncut and edge them in their own shop. The 
amount saved is trifling when the risk of breakage is considered; 
still, many are willing to accept the responsibility. A great deal 
of care should be used in marking these lenses, especially with 
cylinder or sphero-cylinder prisms. 
A piano prism must be accurately ground; that is, the power 
must be perfect, as any decentration cannot change it. When the 
Figure 22 
lens is selected, it is first tested to ascertain if the power is correct, 
either by using a test prism to neutralize it, or by sighting at a 
prism chart (Figure 22). This consists of a series of lines, an 
equal distance apart, graduated from %° to 30°. At whatever 
distance the chart may be arranged for, the first long line appears 
to be moved along the chart, indicating the power of the prism 
that is tested. This method is convenient, but is used only with 
piano prisms. 
A piano prism will appear to throw the line to one side, the 
same as any lens out of center. When twisting a piano prism, the 
line does not remain stationary, as with a spherical, or twist, as 
does a cylinder. The movement appears to move the line to, or 
away from, the straight line, but always parallel to it. To experi- 
ment, take a prism, hold it with the base up or down, and sight 
at a straight line; this you will find will be continuous. Now turn 
the prism, and the straight line will move to one side, until a 
38 ME CHA NICA L OP TICS 
39 
quarter turn has been made; this will be the extreme point that 
the line will travel, and will indicate the power of the prism. Now 
continue in the same direction, until another quarter turn has 
been made, and you will notice that the line will appear to move 
back until it is continuous again (Figures 23, 24 and 25). In 
neutralizing, the test prism is placed against the one to be tested, 
and when looking at a straight line, it will be continuous. 
When marking a piano prism, it is held so that the base will 
either be upward or downward. We then sight at a straight line 
in the same manner as a compound, but in order to make the line 
continuous it must be rotated. Moving the lens sideways will 
Marking Plano Prisms 
Figure 23 
not change the lens in any respect. If the lines are not continuous, 
there will be a prism power either base upward or downward, as 
well as in or out. 
Having lined up the lens correctly, a mechanical axis or cutting 
line is drawn from base to apex, provided the lens is to be fitted 
with the base in or out. If the base is to be up or down, it must 
be lined up and dotted in the same manner, but the mechanical 
axis is drawn at right angles. As there is no center on a piano 
prism, it does not matter whether it is cut in the center of the 
lens, or near the apex or base, except that the thickness will vary. 
If a thin lens is desired, it can be cut near the apex, or thin edge; 40 
MECHANICAL OPTICS 
if a thick one, cut it near the base, or thick edge (Figure 26). The 
one point to bear in mind is, that the power does not change as 
you move the pattern by which the lens is cut up or down, in or 
out, provided it is not twisted. 
If a piano prism is twisted, we immediately obtain power in 
the opposite direction (Figure 27). A chart is published for the 
purpose of computing double prisms. This is not only convenient 
for marking these lenses, but it also tells us the amount of prism 
Figure 24 
Figure 25 
obtained in both directions, when a prism of any diopter is fitted 
at any angle other than 90° or 180°. For example, if a 2A prism is 
fitted at axis 45°, it will have the effect of out and up; 
or if the prescription calls for 2A in and 2A up, we can cut a 3A at 
axis 135°. By this method it will be easily seen that a slight 
variation will produce a prism power in the opposite direction. 
Sphero Prisms 
A sphero prism is handled in much the same manner as a piano 
prism, except that there may possibly be a center on the lens, and 
this must be considered. A weak spherical with a high power 
prism will have no center on the lens, whereas a strong spherical 
with a low power prism will have the center so near the middle 
of the lens that the prism power can be cut or ground out very 
easily, and a simple spherical produced. A sphero prism is noth- MECHANICAL OPTICS 
41 
ing more nor less than a decentered spherical, and by referring to 
the rule it will be seen that if a spherical lens is decentered it will 
produce 1A prism power for every 10 mm. decentration. If spher- 
icals were large enough, it would not be necessary to grind sphero 
prisms, but to produce the high powers it would require immense 
blanks. A spherical can be decentered a limited amount, and 
tables are published by the prescription houses for this purpose 
(Figure 28), but it is impossible to decenter any lens much more 
than 2 mm. Extra large lenses can be used, to be sure, but these 
also cost extra, so that it is just as cheap to use sphero prisms. 
Figure 26 
Figiire 27 
In marking, the lens is lined up the same as a piano prism; that 
is, with the base up or down, and the mechanical axis drawn as 
before. It is then turned in the opposite direction, and a test 
prism held over it, of the required power. A short cross line is 
then drawn at the center. In cutting, the lens must be placed on 
the machine so that the cross line comes exactly in the center 
of the pad, otherwise the prism power will be reduced or in- 
creased. By this it will be seen that a sphere prism cannot be 
treated in the same manner as a piano prism. It is possible, how- 
ever, to twist or rotate the lens, so as to produce a prismatic 
power in the opposite direction, and double prisms can be obtained 
by the use of the chart, as before described. 42 
MECHANICAL OPTICS 
Cylinder Prisms 
A cylinder prism is more difficult to mark, especially if the axis 
is oblique. Also when the base of prism is at any other point 
than at vertical or horizontal. For example, we will first select 
the simplest form, + 1 axis 90 O in or out. This lens will have 
no power in the vertical meridian, or on the line of the axis. 
There also should be no prismatic power in this direction if the 
lens is ground correctly. If there is a prism up or down, the lens 
Diopters 
1A 
2A 
0.50 
4.7 
0.75 
3.1 
1.00 
2.3 
4.7 
.7 
9.4 
1.25 
1.8 
3.7 
5.6 
7.5 
1.50 
1.5 
3.1 
4.6 
6.2 
1.75 
1.3 
2.6 
4. 
5.3 
2.00 
1.1 
2.3 
3.5 
4.7 
2.25 
1. 
2. 
3.1 
4.1 
2.50 
.9 
1.8 
2.8 
3.7 
2.75 
.8 
1.7 
2.5 
3.4 
3.00 
.78 
1.5 
2.3 
3.1 
6.2 
3.25 
.73 
1.4 
2.1 
2.8 
5.7 
3.50 
.67 
1.3 
2. 
2.7 
5.3 
4.00 
.58 
1.17 
1.7 
2.3 
4.7 
4.50 
.52 
1. 
1.5 
2. 
4.1 
5.00 
.47 
.94 
1.4 
1.8 
3.7 
5.50 
.42 
.85 
1.2 
1.7 
3.4 
6.00 
.39 
.78 
1.1 
1.5 
3.1 
Figure 28 
must be reground, for it cannot be decentered in this meridian, 
as it has the same effect as a piano prism. In the horizontal 
meridian this lens has a power of + 1, and, by decentering, the 
prism power can be increased or decreased, the amount, of course, 
depending on the size of the lens to be cut. With a stronger MECHANICAL OPTICS 
43 
cylinder, the amount could be considerable, so that one can 
readily see the necessity of accuracy. 
It must be understood that prism power and decentration are 
the same; that is, by decentering a lens, we produce prism power. 
Some refractionists seem to have an erroneous idea regarding 
this matter, as often a customer will object to having prisms 
ground, and states that he wants decentered lenses. If a pre- 
scription is ordered with the lenses decentered in 2 mm., perhaps 
on account of a narrow P.D., a prism is produced in the lenses, 
although the original idea was to obtain the correct pupillary 
distance. If this were thoroughly understood by every refrac- 
tionist, it would make the work much easier for those who fill 
their prescriptions. 
Marking the Axis 
To return to the lens in question, we find that the lens must 
have no prism power in the vertical meridian. This having been 
found correct, we proceed to dot the axis. There are two ways 
to do this-one is to line up the lens at right angles to the axis, 
and the other is to place the test prism on the lens to be marked 
and dot the axis just as if there were no prism on the lens at all. 
Figure 29 
Figure 30 
By the first method, we line up the lens so that the line is con- 
tinuous, and place a dot at each edge. This will be the mechanical 
axis, or cutting line. This line can then be drawn. We then 
select a 1° test lens, and hold it over the lens to be marked, and 
center the lens in this direction, placing a short cross line at the 
point where the lines intersect. By this it will be seen that if the 
prism power is weak, the cross line will be moved towards the 44 
MECHANICAL OPTICS 
apex, or thin edge, and if strong, towards the base, or thick edge, 
which shows that the prism was not accurately ground. It also 
shows us that a lens ground with a 1° prism could be decentered 
to produce or 1 %A, although in reality the blank would not 
be large enough to cut an ordinary size lens, unless the focus was 
strong enough (Figure 29). 
If the second method is used, the test prism is selected and 
placed over the lens to be marked, with apex to base, in order to 
neutralize the prism. We then line up the axis, as with a piano 
cylinder, dotting it at the edges. It is then laid on a card with 
cross lines, and a cutting line or mechanical axis is drawn at right 
angles to the axis. As there is no center in this meridian, it does 
not matter where this line is drawn as long as the size of eye can 
be cut out (Figure 30). 
We will now suppose that the lens was + 1 axis 90 O 1° up or 
down. The cylinder would have no power in the vertical merid- 
Figure 31 
Figure 32 
ian, but would in the horizontal. In this combination, the prism 
power must be accurate, as there is no possible way to change it 
by decentration. As there is a cylinder power in the opposite 
direction, we can produce a prism by decentering the lens in or 
out (Figure 31), so that if care is not used we will get a double 
prism. In marking a lens of this description, either of the two 
methods before described can be used. We first test it with a 
prism, to ascertain if the power is correct. The lens will then be MECHANICAL OPTICS 
45 
lined up on the axis, in the same manner as a piano cylinder. If 
the lens has been ground inaccurately, so there is a little prism 
base in or out, the axis dots will come a little to one side. This 
will not matter, however, as long as the size required can be cut 
(Figure 32). 
The lens is then placed on the cross lines, and the cutting line 
drawn at right angles. It is of no consequence whether this line 
is in the middle of the lens or not, as far as the prism power is 
concerned, but it will make a difference in the thickness of the 
lens. If it is cut near the base, it will be thick; if near the apex, 
thin. If the lens, when surface-ground, is left full size, it can be 
cut fairly near the apex, as it will be quite thick enough for the 
frameless lens. A prism with base up or down can be used con- 
siderably thinner than one in or out, for the reason that in the 
first form the straps are fitted in midway between the base and 
apex, whereas in the second form one strap, if a spectacle, must 
be fitted to the apex, or thin edge, and if an eyeglass, the lens can 
also be used thin, provided the base is in, because there is no strap 
to be fitted on the thin edge, except possibly a handle, and now- 
adays these are rarely used. 
If the lens is to be marked, using the second method, test the 
prism power, and line it up at right angles to the axis, placing the 
dots on the edge. It is then laid on the card and the mechanical 
axis drawn. We then hold the lens in the hand, and sight at a 
line; by moving the lens until the line is continuous, we find the 
center, which in reality is the axis. In both these methods, with 
the base up or down, it must be remembered that the prism 
power must be ground accurately, and tested with a test prism 
before attempting to mark it, as the prism power cannot be 
changed. With the base in or out, the prism power can be changed 
slightly by decentration, as described. 
Cylinder Prisms With Oblique Axes 
Cylinder prisms, with the axes oblique, are more complicated; 
consequently, are more difficult to mark. If the base of the prism 
is at 90° or 180°, the prism power is easily neutralized, and the 
lens is then marked as if it were a piano cylinder. If the prism 
power is accurately ground, it simplifies the work considerably, 
as there are no extra conditions to deal with. There are many 46 
MECHANICAL OPTICS 
lenses of this nature in use today that are not correctly ground, 
and yet they have been marked and cut in ignorance of the fact 
that there is prism power up or down when not called for on the 
prescription. 
Everyone should always take pride in the work he sends out, 
for when repairs are to be made your customer may take them 
elsewhere, and as your case bears your name your reputation may 
suffer. Those who prescribe many prisms usually understand 
them, and are quite particular; so it pays to be accurate in this 
work. A very easy way to handle this work is to scratch the axis 
on the rough cylinder, before grinding the prism, but this is very 
apt to cause breakage when heating the glass in blocking. If 
there is only an occasional lens to be ground, it is possibly just 
as well to take a chance, but in the large prescription shops break- 
age can be quite an item, so these lenses are usually ground with- 
out scratching the axis. 
Figtire 33 
We will take, for example, + 1 axis 45 O 1A out, and suppose 
that the prism is accurately ground-select the 1A test prism and 
place it over the lens to be marked with the apex to base. The 
prism is then neutralized, and the lens is practically a piano 
cylinder. Now turn both lenses so the axis can be lined up, which 
will bring them cornerwise (Figure 33), provided the cylinders are 
ground this way. After dotting the edges remove the test lens, 
and lay the lens to be marked on the axis card, with the dots at 
45°. Then mark the cutting line or mechanical axis in the middle 
of the glass, but without a cross line for the center (Figure 34). MECHANICAL OPTICS 
47 
Now take the test prism and place it in position as before, and 
sight at a straight line. This will appear twisted, as in Figure 35, 
and if the prism power is correct, the line will be cut top and 
Figtire 34 
bottom an equal amount. If the lens is not held in the correct 
position, or if the prism is not correct, it will be cut more at one 
end than at the other (Figure 36). By moving both lenses from 
side to side, until a point is found where the lines break evenly, 
Figure 35 
Figure 36 
a center will be found. If this is at one side of the middle of the 
lens, the prism power is strong or weak, and if the size cannot 
be cut, it must be reground and corrected. 
In marking cylinder prisms it must be remembered that the 
lens cannot be twisted, but must be cut as ground, as turning the 48 
MECHANICAL OPTICS 
lens to change the axis turns the prism also, thus making a double 
prism. Turning the lens to make the prism correct will bring the 
axis off at the same time. 
A cylinder prism with the axis oblique, and the base up or 
down, is marked in the same way, but care must be used so it is 
not decentered, thus producing a prism in or out. In all weak 
power cylinders the decentration, when the axis is oblique, does 
not amount to very much, and possibly will do no harm to the 
wearer, but the higher powers require only one or two millimeters 
decentration to produce quite a little prism power. With the base 
in this direction, it is well to cut it as near the apex as possible, 
and if the center comes fairly low it is all the better (Figure 37). 
When the axis of the cylinder is oblique, and also the base of 
prism, it really produces a double prism, and it should be marked 
as such, unless the prism is at the same axis as the cylinder. For 
example: + 1 axis 45 O 2A axis 45 would be either + 1 axis 45 
Figure 37 
Figure 38 
O 1^2a out O 1^A up, or + 1 axis 45 O in O 1XA down 
in the left eye. If this combination were to be fitted in the right 
eye, it would be + 1 axis 45 O in O up, or + 1 axis 
45 O °ut O down. By this you will see that it makes 
considerable difference to which eye the lens is to be fitted. If a 
lens were ground for the right eye + 1 axis 45 O 1^A out O 
1J/2A up, it could be fitted base in and down, or if to the left eye, 
it would be in and up, or out and down. 
These points must be kept in mind when the lens is being 
ground, also when marking, as it is a very easy matter to cut them 
wrong. In the combination referred to, + 1 axis 45 O 2A axis MECHANICAL OPTICS 
49 
45°, the lens would first be lined up on the axis just as though it 
were a piano cylinder, and in this direction the lines would be 
continuous; in the opposite direction the line will be broken, as 
in Figure 38. 
A 2A test prism should then be held over the lens to be marked 
with the apex to base, and when holding the two lenses with the 
axis at 180°, the lines should be continuous (Figure 39). If the 
lenses are held with the axis at 45°, the lines should be cut evenly 
at the top and bottom (Figure 35). Either way is correct, but a 
dot must be placed in the center, otherwise the power of the 
prism will be changed. When the three dots are in position, that 
is, the two at the outer edges, and one in the center, the lens is 
then placed on the axis chart at 45°, and the cutting line drawn. 
In a combination with the base of prism at right angles to the 
axis, for example, + 1 axis 45 O 2A axis 145, the lens is first lined 
up across the axis, as at this point the line will be continuous, 
whereas the line will be cut on the axis. 
Figtire 39 
Figure 40 
In a combination with the axis of the cylinder, and the base 
of prism oblique, but at different axes, the marking is quite diffi- 
cult. For example: + 1 axis 45 O 2A axis 25. In this case we 
have a double prism, and if a prism chart is at hand, we can locate 
the combination very easily and treat the lens as such. This com- 
bination would have the effect of about in O 1° up in the 
right eye, and would appear when held with the axis at 45°, as 
in Figure 40. By this you will see that the lines are cut or twisted 
in both meridians, and of unequal amounts. With the proper 
test prisms, and placed in the correct position, the lines will be 50 
MECHANICAL OPTICS 
cut evenly. In a complicated lens of this description it is better to 
mark the axis of the lens, or the cutting line, with a diamond 
before surface-grinding, and in this way it is only necessary to 
neutralize the prisms. 
Compound Prisms 
Compound prisms are dealt with in much the same manner as 
cylinder prisms, except that one must bear in mind that there 
is power in both meridians, and any decentration affects the 
power of the prism. Any combination with a low power prism 
will have the center at some point on the lens, so in reality it is 
really a decentered compound (Figure 41). It is simply necessary 
to place this center in some position to produce the necessary 
Figure 41 
prism effect. This is the theory of a compound prism, and the 
only reason any desired amount of prism cannot be produced is 
that the lens is not large enough. If one will keep this point in 
mind, the handling of these lenses is a very simple matter. 
The trouble that most opticians have is due to the fact that 
they seem to have an idea that a prism is some mysterious ele- 
ment. Take for example: + 10 + 1 axis 900 1A out. This is 
one of the simplest forms, and in sighting through a lens of this 
combination we find that it is centered in the vertical meridian, 
and that in the horizontal meridian the center is at one side. 
With a + 2 spherical we find by referring to the rule, that the 
center is five millimeters from the line of the axis. As there is a 
power of 2 D. in this meridian, we would by decentration of 5 
mms. obtain alA. As this is more than the size of the lens to be 
cut allows, it is necessary to grind the prism-or in other words, 
we grind a lens + 10 + 1 axis 90, but with the center five milli- 
meters to one side. When we come to mark the lens, if the full MECHANICAL OPTICS 
51 
amount of prism were not ground, we can decenter it one or two 
millimeters either way, to produce the full amount. For example: 
Suppose the lens when coming from the surface department had 
the center millimeters from the axis line, we would have then 
but about prism; by decentering the lens more we would pro- 
duce the 1°. If the lens were decentered up or down, we would 
also produce a prism in this direction, thereby making a double 
prism. 
With a cylinder prism, having a combination of + 1 axis 90 
OF out, it would be possible to cut the lens at any point in the 
vertical meridian without changing the power in any way. This 
is explained in full in the preceding pages. It will be noted, how- 
ever, that with a compound prism, it is impossible to do this, and 
that the lens must be cut at one point on the vertical meridian. 
If the lens should be ground with a slight prism up or down, it 
could be decentered to cut this out. 
In marking this combination, first line it up at right angles 
to the axis, place a dot at each edge of the lens, and this will be 
the cutting line, or mechanical axis. Then select a test lens 
and hold it over the lens to be marked. We then sight at a straight 
line and bring the lens in center. At this point place another dot, 
and this will be the mechanical center; in other words, this point 
must be placed in the center of the pad on the cutting machine. 
If a pattern is to be used to cut by hand, place the center of the 
pattern on this point. After the lens is cut, it will be seen that 
this is not the true center; this will be at one side. If we are not 
accurate in placing the lens on the pad or pattern, we can easily 
lose some of the prism effect. 
A sphere cylinder prism, with the axis at 90 or 180°, combined 
with a prism power, base up or down, is just as simple to mark as 
a combination with the base in or out. For example, take + 1 O 
+ 1 axis 90 0 1A up. This will have the effect of a compound 
with the combination -J- 1 O + 1 axis 90, but the center would 
be ten millimeters above the middle of the lens. In marking, 
we would first line up the lens, finding the axis just as though it 
were a regular compound. In the vertical meridian the line 
would be continuous; place a dot at each edge of the lens; the 
center one cannot be marked at this time. 
We have now located the axis, and the next step is to make 
the cutting line, but before this can be done it must be located 52 
MECHANICAL OPTICS 
with a prism. Select the 1A test prism, and place it next the lens 
to be marked, and line it up at right angles to the axis. When 
the lens is in position, so that the line is continuous in this direc- 
tion, place a dot at this point, in line with the two dots marking 
Figure 42 
the axis. Now place the lens on the axis chart and draw the 
cutting line. A short cross line should also be drawn in line with 
the axis dots, to obtain the center in this direction. Figure 42 
shows the position of line in opposite direction to axis before 
placing prism in position. 
Figure 43 
As the rough lenses are at least 44 mm. from corner to corner, 
there is quite a little room for decentration (Figure 43). It should 
always be remembered that the center of a convex lens with a 
prism will always be toward the base, and that the center of a MECHANICAL OPTICS 
53 
concave lens with a prism will be towards the apex. In this 
combination just referred to the center will, of course, be up. 
With a combination 4- 1 + 1 axis 45 O 2A out, the horizontal 
Figure 44 
line will be cut equally (Figure 44), but the vertical will be 
unequal (Figure 45). First place a 2" test prism next the lens to 
be marked to neutralize the prism power, then line up the lens on 
the axis, just as though it were a regular compound lens. At this 
time, center the lens in the opposite direction, also, and place a 
dot at this point. Now lay the lens on the axis chart and draw 
the cutting line. After marking the lens, it can be verified by 
Figure 45 
sighting at the cross lines; these will appear as in Figure 45. Now 
place the test prism in place again and it will be noted that the 
lines are cut equally in both meridians. 54 
MECHANICAL OPTICS 
A combination of + 1 O + 1 axis 45 O 2A axis 45°£will*be a 
double prism, just the same as in a cylinder prism, and should 
be treated as such. When held at axis 45° the lines will appear 
Figure 46 
Figure 47 
cut unequally in both meridians (Figure 46). By turning the lens 
to 90°, to line it up, it will be noted that it has the effect of a 
regular sphero cylinder, in this meridian; therefore, the axis is 
easily lined up. 
After placing the dots on both edges, take the /test prism and 
place it next the lens to be marked, with the apex exactly in line 
with the axis. Now sight at the cross-line, and move the lenses 
until the vertical line is cut equally at top and bottom (Figure 47). 
Place a dot at this point and we have the three dots in position 
to mark the cutting line. Now lay the lens on the axis chart, 
draw the cutting line, and make a short cross-line for the center. 
It is absolutely necessary to mark this center, otherwise the 
power of the prism would be changed if the lens were cut at some 
other point on the mechanical axis. 
A combination 4-10 + 1 axis 45 O 2A axis 135 would appear 
as in Figure 46, both lines being cut unequally. When turned 
to axis 90, as in the previous example, it will not line up, but the 
lines will be displaced as in a piano prism (Figure 48). In this 
case it will be necessary to line up the lens at right angles to the 
axis. In this direction the line will be continuous. Place the dot 
on the edges in the regular way and then place the test prism in 
position with the apex at 135°. Now sight at the vertical line 
and move the lenses until the line is cut equally at top and bot- MECHANICAL OPTICS 
55 
tom, and place a dot at this point. The lens is then laid on the 
axis chart, and marked as before. 
Figure 48 
With a combination of + 1 O + 1 axis 45 O 2A axis 25 the 
lines will be cut in both directions, and as this is somewhat com- 
plicated it is well to mark the axis of the prism with a diamond 
before surface-grinding. The reason for this is the difficulty to 
place the test prism in the correct position, and some guess- 
work is necessary. This combination also has the effect of a 
double prism. 
In grinding all combinations of double prisms, it should be 
remembered that the correction can be divided between the two 
eyes, and still have the same effect. For example: If the pre- 
scription calls for R. + 1 O + 1 axis 90° O 1A up and 1A out, 
L. + 1 O + 1 axis 90° O 1A down and 1A out, we could grind the 
lenses R. + 1 O + 1 axis 90° O 2A up, and L + 1 O + 1 axis 
90° O 2A out. This simplifies the grinding to some extent, but 
the disadvantage is that we do not produce lenses of even thick- 
ness. It must be remembered, however, that a prism base out in 
one eye, and base in in the other has no prismatic effect, as one 
neutralizes the other. This also applies with prisms base up and 56 
MECHANICAL OPTICS 
down; that is, two lenses placed in a frame, base up, have no 
prismatic effect, and when dividing the prism power, one must be 
base up and the other base down. 
We rarely see mistakes made with prisms in or out, as the pre- 
scription reads the same. For example: It is correct to write a 
prescription + 10 + 1 axis 900 la out OU, and in this case 
the prism power of 2A is equally divided between the two eyes. 
If we should write a prescription + 10 + 1 axis 90 O 1A up, 
OU, it would be wrong, as both prisms would be in the same direc- 
tion, and would neutralize each other. The correct way to write 
this prescription is: Right, + 10+1 axis 90° OP up, and 
Left, + 1 O 1 axis 90° OF down. 
Test prisms for neutralizing should always be marked with a 
diamond on the apex and base, also at right angles on both sides. 
When the lenses are marked this way we are always sure that they 
are placed in position correctly. 
Marking Bifocals 
Some prefer to purchase these lenses uncut, and edge them in 
their own shop. In marking these lenses, a crayon pencil kept 
sharp may be used in place of ink, if any marks are to be placed 
on the disk. For this purpose a glass pencil, such as is used for 
marking china, is very convenient. It is blue in color, and is very 
easily distinguished when laid on the axis chart. 
In marking these lenses, the first step is to make a series of dots 
around the edge of the disk, so that the outline can be distin- 
guished (Figure 49). We next line up the distance part of the 
lens, just as though it were an ordinary lens. If this is a spherical, 
we need only to find the center and at this point we place a dot 
or mark a cross, as you prefer. We now lay this on the axis chart, 
with the disk in the center, if it is to be in this position, and draw 
a cutting line. 
For lenses of this character, an axis chart as shown in Figure 
50 is a great convenience. Charts of this type are supplied by 
several manufacturers. It will be noted that the wafer is indi- 
cated two millimeters below the center of the chart. This is the 
usual position of the reading portion on all lenses, and unless 
otherwise ordered, a disk or wafer, if a cement bifocal, should be 
placed two millimeters below the center of the distance lens. It MECHANICAL OPTICS 
57 
is always well to consider this point, however, for if the glasses 
are to be worn constantly and the patient desires to read very 
little, a small disk can be used. 
Figure 49 
On the other hand, if they are to be used constantly for near 
work and the patient desires only to glance up occasionally, a 
larger disk is more desirable. In a case of this kind, the disk is 
brought up to the center of the distance lens, and sometimes 
above. As twenty millimeters is the average size, and two milli- 
meters below center the usual position, it is best to make the chart 
this way, and make the necessary allowances for the occasional 
conditions. The circle will be used for cases where the disk is to 
be in the center; and for jobs where the disk is to be set in one or 
two millimeters, use the lines either side of center. 
For a Right lens, we will use the line at the left, and for a Left 
lens, the one at the right. These are drawn for one millimeter 
displacement. 
In marking a spherical lens it is only necessary to place the 
center of the distance lens on the center of the chart, and twist 
the lens until the disk coincides with the line in whatever position 
you desire to place it. A cutting line or mechanical axis is then 
drawn, and the lens is ready to cut. In cylinders or compounds, 
the disk is dotted as before described. The distance lens is then 
lined up for the axis. This is dotted in the usual way, but always 
select the meridian that does not pass through the disk, if pos- 
sible. For example: If the lens is to be axis 90°, it is better to 58 
MECHANICAL OPTICS 
line it up on the 180° line first, as we can then sight away across 
the lens. If we line it up at 90° the disk interferes. 
Figure 50 
When the axis has been located and dotted, lay the lens on the 
axis chart, with the dots at the proper axis, and the disk over the 
line that is in the correct position. It will be noted that this lens 
cannot be twisted the same as a spherical. In twisting a lens of 
this kind, either the axis or the disk will be thrown out of position. 
We can, however, move the lens to one side in order to bring the 
disk in the correct position, without changing the axis. 
In laying the lens on the chart, notice at all points if the lens 
is correct. First the axis, then the location of disk, in or out, and 
also if top of disk is the correct distance below the center of the MECHANICAL OPTICS 
59 
distance lens. If all points are correct, draw the cutting line, or 
mechanical axis. If for any reason the axis is not correct, the 
cylinder can be reground and corrected, provided the lens is not 
too thin. 
Figure 51 
Also notice that the disks on the two lenses match; that is, do 
not get one on center, and one set in, or one higher than the other; 
as this will make the disks appear different in shape and size 
(Figure 51). Be particular also,when the wafers are to beset in, 
that you do not make a mistake, and mark both lenses for one 
eye. If this should happen, you will see after cutting that one 
disk will be in and the other out. This will necessitate the grind- 
ing of another lens. As the disks on these lenses are smaller than 
the regular cement size, and also as the eyes converge looking at 
a near point, according to theory, they should always be set in 
at least one millimeter. 
Marking Cross Cylinders 
Cross cylinders, with the axes at right angles, are just as simple 
to mark as compounds, but with the axes at any other angles, it 
is very difficult to find the correct axis. In preparing lenses of 
this character to be surface-ground, they should always be 
scratched with a diamond. 
A peculiar feature about these lenses prevents their being lined 
up the same as any other lens. This is the fact that they always 
line up at right angles, no matter where the axes are. For 
example: + 1 axis 25 O - 1 axis 90 would have the axes accord- 
ing to the prescription 65° apart, and yet in reality they would 
line up at 90° apart just the same as any compound. The true 
position of the lines would be at axes 12° and 102° (Figure 52). 60 
If we could transpose a combination of this kind mathemati- 
cally in a few seconds we would know just where to line it up. As 
this is impossible, and in fact only a very few can work them out 
MECHANICAL OPTICS 
Figure 52 
any way, it is much better for us to depend on our axis marks. 
The simplest method of transposing cross cylinders is to take two 
cylinders and, after marking the axis of each, place them together, 
and then line up the combination. If one finds it difficult to hold 
them without slipping, the two plane surfaces can be cemented 
together. If for any reason a cylinder is surface-ground, and the 
diamond marks were omitted, the simplest way to locate the axis 
is with the lens measure. First locate it as near as possible, and 
then place dots on both edges. Then take the lens measure again 
and place the points in line with the axis, and by twisting it a few 
degrees either way, and noticing the movement of the pointer, 
the axis can be found within one degree. There is an instrument 
on the market, known as the Axis Indicator, which is used in con- 
junction with the lens measure, and is invaluable for this work. 
Cement Bifocals 
In selecting stock for bifocals the upper lenses are marked as 
usual with one exception, and that is, the wafers are always 
placed on the inside when possible. For sphericals, periscopic 
lenses can be used for the uppers and "regular" wafers cemented 
on the inside. For example: Prescription distance + 2, reading MECHANICAL OPTICS 
61 
+ 4; select + 2 Pcx. and a "regular" wafer, having a + 2 power, 
which is the necessary addition for reading. This will be + 1.25 
on one side and + .75 on the other. The + 1.25 side is cemented 
Figtire 53 
to the - 1.25 curve on the upper lens. The + .75 on the outside 
of the wafer and the + 3.25 on the outside of the upper lens will 
then give us the required focus (Figure 53). The contact surfaces 
are not considered in any way, other than that they must fit per- 
fectly, as they have nothing whatever to do with the focus of the 
lens when the index of the glass is the same. If it is different, as 
in fused bifocals, it is then an important matter, and must be 
taken into consideration. 
For cylinders piano wafers are used and cemented on the piano 
side. You will have no trouble with these, as the power of the 
wafer is all on one side. Be careful, however, never to cement 
a wafer on a cylinder surface, as the cylinder effect is then de- 
stroyed. Compounds are a little more difficult, but are simple 
when understood. The wafer is usually placed on the inside in 
weak combinations. The only reason for this is to make them 
more invisible. Whichever way they are made the wafer will have 
to be cemented to the spherical surface, and here is where you 
will, perhaps, be confused in marking. Be sure that you lay the 
lens on the protractor, with the surface upward, that is, to be 
next the eye. If the spherical is to be in, mark it on the spherical 
side. If the cylinder is iw, mark on the cylinder side. You can 
readily see that if the axis is to be 45°, with the cylinder surface 
out, and you mark it on the cylinder side, and then turn it over, 
the axis will be 135°. 
In selecting the wafers the contact surface will be the same 
power as the spherical surface on the compound, but the curve 
will be the reverse. To this power add the amount to be added 62 
MECHANICAL OPTICS 
for reading and you will have the outside surface. For example: 
Prescription + 1 O + .50, axis 90°, distance add + 2 for reading. 
We will first select - 1 to fit the + 1 surface, and by adding the 
+ 2 for reading we have + 3. Then the wafer wanted is - 1 O 
+ 3. 
Lens Cutting 
You should first become familiar with the working of your 
machine. Read the directions and be sure that you understand 
how to set it for size. Some of them are set by charts; that is, 
for length and width. Others are set by the difference between 
the length and width. In other words, the patterns are marked 
from 4 to 12, so that 40 x 33 would require No. 7 pattern, and the 
gage is set at the length, 40 mm. In cutting for frameless, allow 
1 mm. on the length and width. For above size it should be cut 
41 x 34. For frame size, it is best to cut a lens and try it in the 
frame, allowing about % to 3/16 inches between the end piece, 
according to the thickness. After you have ground one or two, 
you. can judge for yourself just the amount you will need for 
grinding. 
Figure 54 
When the machine is in position to cut, the handle on the top 
gear should be at the right. It should always be turned to the 
left, making one revolution, and stopping at just the point where 
it started to cut. Never, under any circumstances, run by this MECHANICAL OPTICS 
63 
point, as you will spoil the diamond. If, for any reason, you do not 
get a satisfactory cut it is better to turn it over and try again on 
the other side. 
In placing the lens on the pad see that it fits perfectly flat and 
does not rock. If you have a universal pad it will be all right; 
but if your machine has a number of pads select the one nearest 
the curve of the lens. Also cut on the surface having the least 
power on the nearest to piano. In laying the lens on the pad, the 
cutting line should be on the 180° line on the cutter, with the 
center on or under the centering pin, whichever way the machine 
is constructed. 
When the lens is in position, raise it to the diamond by pressing 
the hand lever on the left side. Start gently at first, increasing 
the pressure gradually, turning the lens slowly at the same time. 
Some diamonds require more pressure than others, but you must 
Figure 55 
experiment carefully until you find that you get a perfect cut 
(Figure 54). This you will see has a sharp line which runs almost 
through the lens. If you put too much pressure on it you will 
have a scratch (Figure 55). This lens you will find very difficult 
to break down, and probably break over the mark, unless you are 
extremely careful. 
There is another way to cut a lens without marking; in this 
way you turn the pad in the machine so that the line corresponds 
to whatever axis you are to cut on the graduated scale. The lens 
is then placed with the axis dots on the line on the pad. This will 64 
MECHANICAL OPTICS 
give you the same result, and, although it saves time, it is not so 
accurate. 
Strong lenses and high curve torics can be cut on any of the 
modern cutters as well as weak ones. If you wish to cut them by 
hand they should be marked in the same way, selecting a brass 
pattern the exact size of the frame or the dimensions of the frame- 
less lens to be ground. Lay this on the lens with the three holes 
over the dots, or on the cutting line, holding it in the left hand 
with your thumb on the pattern. Use the diamond in the right 
hand in a vertical position, the same as a pencil, or with the han- 
dle between the index and third fingers. Cut lightly around the 
pattern and be careful not to run over the line. CHAPTER V 
Cutting Complicated Lenses 
COMPLICATED or expensive lenses should be cut by hand 
in some cases, and although many are cut by machine, 
one should be sure that every part is in perfect working 
order. One of the most essential things is to see that the pad on 
which the lens is laid fits the under surface of the lens perfectly. 
It does not matter so much about the center, unless the lens is 
Figure 56 
very thin; in this case care will have to be used to prevent the 
center pin, which keeps the lens from slipping, from punching 
through the center. 
In selecting a pad to fit a lens, notice if it has a good bearing 
under the part where the diamond travels. If the lens has a deep 
Figure 57 
curve, and the pad is shallow, the point of contact will be in the 
center, and there will be no bearing under the diamond. In a 
case of this kind, the lens has a tendency to rock; this is not only 
liable to cause the lens to slip, but even if the diamond cuts at 
all, the best results will not be obtained. Figure 56 shows a lens 
on a pad too deep; Figure 57, one too shallow; Figure 58, the cor- 
rect curvature. These illustrate toric lenses, or lenses with strong 
outside curves. 
65 66 
MECHANICAL OPTICS 
Flat lenses should always be placed on a flat pad. Many 
opticians neglect to change their pads, however, and although a 
Figure 58 
flat lens could be cut on a pad having a slightly concaved surface, 
it would be impossible to get good results on a pad having a con- 
vex surface, as the lens would have a tendency to tilt (Figure 59). 
Figure 59 
A very good pad is now furnished with some machines, that 
might rightly be termed "A Universal Pad." This consists of a 
flat rubber pad having a raised oval-shaped center (Figure 60). A 
Figure 60 
flat lens when lying on this pad will rest on the top, and as the 
oval is about the average size of lenses cut, it has a very good MECHANICAL OPTICS 
67 
bearing. A convex lens rests on the inside of the oval, and as 
the center of the pad is cupped out, it does not touch at this point. 
A concave lens rests on the outside of the oval, and as the pad 
is flat outside of the raised part, it allows room for the thick edges 
to overhang (Figure 61). This pad can also be used for toric 
lenses of ordinary curve. 
Having made sure that the lenses rest properly on the pads, 
then notice if the foot that presses in the center is adjusted cor- 
rectly. Some machines have a screw adjustment which allows 
Figure 61 
it to be regulated to the proper tension, and others work entirely 
by spring tension. The points of contact also vary; some are 
just a rounded metal surface, and some have a ring about 
diameter, and still another has three rubber-covered points in the 
shape of a tripod. Whatever style is used, be sure that they are 
adjusted to prevent the lens from changing its position, thereby 
throwing the axis out or else obtaining an irregular shape. Next 
be sure the diamond is cutting in good shape; do not attempt to 
use one that needs resetting, as a poor diamond can easily spoil a 
pair of lenses that would cost more than the cost of resetting. 
Figure 62 
That the formation of a cutting stone may be thoroughly 
understood, we show an enlarged illustration of a point (Figure 
62). This is of natural formation, and cannot be recut. It can be 
reset at a different angle, or if this point has been damaged, a 
new one on the same stone can be used. The best machines are 
supplied with Brazilian sparks, and although African sparks look 68 
MECHANICAL OPTICS 
the same to a novice, they will not produce as good results. This 
point should always be considered when purchasing a machine. 
Figtire 63 
This also applies to hand diamonds. Many opticians have a 
mistaken idea regarding these tools. There are offered for sale 
glazier's diamonds at a low figure, and many times they are pur- 
chased at pawnshops or second-hand stores as low as one dollar. 
A good hand diamond for optical use costs at least ten dollars, 
and is well worth the investment (Figure 63). 
A diamond that has lost its cutting point will only scratch. As 
soon as this is noticed, experiment a little and see if a cut cannot 
Figure 64 
be obtained by using a different pressure. The amount of pres- 
sure required varies, but this does not necessarily mean that the 
best diamonds require light pressure. In fact, one that requires 
a fairly firm pressure is to be desired, as a cut may then be forced 
away through the glass. 
There is another point regarding the setting of a diamond, and 
that is the angle. A diamond may cut properly in a straight line, 
but when used on curves it will not cut at all. We occasionally 
find this trouble with cutting machines, and frequently the lens MECHANICAL OPTICS 
69 
may be cut all right all the way around, except at one point on 
the side (Figure 64). At this point the diamond has a tendency 
to travel away from the pattern, thereby causing a hump on the 
side. This may be prevented in some cases by increasing the 
spring tension, but if it requires more than a reasonable amount, 
or if it does not remedy the difficulty, the diamond should be 
reset. Attachments to cutting machines can be secured which 
enable the operator to keep the point of the diamond at a right 
angle to the cutting surface of the lens. 
There maybe another cause for this,and that is, the micrometer 
head may not swing easily in the arm that holds it. In most 
Figure 65 
machines there is a set screw with a check-nut at the top to regu- 
late the play at this point. This should be adjusted so there is 
no play up and down, but so the micrometer head swings easily. 
A drop of oil at this point will also help a little. If this is adjusted 
so that it works too stiff, the micrometer head has a tendency 
to travel in a straight line, rather than to follow the curve of the 
pattern. 
This theory also applies to a hand diamond, and we End in use 
a great many so-called pencil diamonds. These are simply 
straight handles with a diamond set in the end. They may pos- 
sibly have one cutting point, but as a rule are intended for scratch- 
ing, marking or writing on glass. A good hand diamond has a 
swivel in which the diamond is set, and in this way the point 
follows the curve of the pattern. The best glazier's diamonds 
are also made in this way, but the swivel is very large, and is 
intended to be used with a thick straight edge. 70 
MECHANICAL OPTICS 
An optician uses a diamond around a thin brass pattern, con- 
sequently these will not answer. The regular optician's diamond 
is set so that one side of the stone bears directly against the pat- 
tern, and in this way a close cut is obtained. Rather than use a 
Figure 66 
poor diamond, one can just as well use a steel point. One of these 
can be made very easily out of a rat-tail file, and by grinding a 
short stubby point it will scratch as well as a diamond. Many 
use these for cutting (although really scratching) wafers. If 
one desires good cutting, it can be obtained only with the best 
diamonds. MECHANICAL OPTICS 
71 
After a diamond has been reset, it will be necessary to change 
the size adjustment. Some machines are so arranged that this 
can easily be done, and others require special scales; in either 
case, cut a lens and measure it with a rule. Then work from this 
as a basis, and if you are obliged to make out a chart, do so; but 
if the machine can be adjusted, it is much more satisfactory. 
For breaking down lenses after they are cut, breaking tongs, 
similar to Figure 65, are to be preferred. Many use flat-nose or 
cutting pliers. There are also tongs with straight jaws, but those 
with the side jaw are easiest to handle. These jaws on the inside 
are rough, so that they grip the glass in good shape; other pliers, 
such as cutting pliers, snap the glass rather than break it easily. 
In breaking down lenses, hold the lens in the left hand, using 
the side of the breaking tongs, and start on the end of the lens. 
Do not use the top, as this is somewhat awkward. After breaking 
it down as well as possible, cribbers should be used (Figure 67). 
These consist of a pair of scissors, so to speak, having square iron 
Figtire 67 
pieces on the inside of the jaws. The glass will roughen the iron, 
and gradually wear it away, and at the same time it makes a 
natural shape which seems to take hold of the glass better. A 
workman using these all the time will wear them down to a point 
where they can hardly be used, rather than fit new pieces to the 
jaws in order to preserve the curve in the jaws. After the jaws 
have been worn considerably, they can be turned over, and the 
opposite side used. These pieces are soft-soldered to the jaws, 
and need only to be heated slightly, and will then drop off. The 
end, it will be noticed, is fitted with a screw; this is to allow for 
different thicknesses of lenses. 72 
MECHANICAL OPTICS 
Extra-strong concave lenses, thick prisms and deep torics can 
best be cut by hand. For this purpose thin brass patterns are 
used, which can be curved to fit the surface to be cut. If one does 
not have an automatic machine, the lenses must be cut and 
Figure. 68 
shaped better. With an automatic grinder many of these thick 
lenses can be cut large, and the machine can be set to grind to 
size. This method is used by some opticians for frame work. 
The bevel is then put on afterward by hand. 
Automobile goggles require a great many shapes, and if one 
does not have an assortment of metal patterns, a temporary one 
can be cut out of cardboard. Many of these lenses are made from 
window glass, or in other words, are blown instead of moulded. 
Glasses for stone-cutter's spectacles are also made from this 
quality of glass, and if one has much of this class of work to do, 
it is better to have a special diamond for the purpose, especially 
for machine work. This point should also be kept in mind when 
ordering a diamond reset, for it is possible to reset a diamond 
that will cut perfectly on one kind of glass, and give very unsatis- 
factory results on another. If your diamond is to be reset by any 
other than an optical workman, it is a good idea to furnish him 
with samples of the glass, otherwise he will probably use win- 
dow glass to experiment on, and the best results will not be 
obtained on optical glass. No matter what method or what 
machine you use, it is well to watch an expert, if possible, or at 
least see samples of what can be done by others. In this way you 
can readily see if you are getting all that you can out of your 
machine. If you do not, there is trouble somewhere, either with 
you or with the machine. MECHANICAL OPTICS 
73 
There are many odd-shaped lenses in use at the present time, 
but the majority can be cut by machine. Many opticians do not 
understand that drop eyes (Figure 68) can be made with any' 
Figure 69 
difference between the length and width. Although No. 9 is the 
regular shape for an oval lens, in the drop eye it looks somewhat 
pointed. No. 7 shape (Figure 70) is to be preferred to No. 9, and 
if one desires still more drop, No. 5 makes a good shape. These 
patterns are carried in stock by the factories, with proportions 
Figtire 70 
ranging from 4 to 10, and if it is possible to use them on your 
machine, it will save considerable work, as well as time, to have 
them. 
Clerical eyes cannot be cut by machine on account of the Hat 
top, and also because usually the width is very narrow. The most 
common sizes are 40 x 20 and 38 x 18. These are also made with 
a crescent-shaped top. All these styles have to be cut by hand. 
Colonial shapes because of the corners cannot be cut on a machine. 
There are also many lenses in use that have what is commonly 
called "full ends" (Figure 69). These run slightly more toward a 74 
MECHANICAL OPTICS 
square shape than the regular ovals. Patterns for your machine 
can also be obtained for this class of work although if your 
machine has a fixed pattern that cannot be changed, these can 
be cut by hand. 
A great many opticians specialize on special shapes of this kind, 
on account of the advertising feature, as well as the desire to be 
exclusive. In a small city or town having four or five opticians, 
competition is very apt to be keen. If one of these grinds a 
special shape it may be possible that his competitors may not 
have grinding plants, and in this case they are unable to use a 
stock lens, but will be obliged to send it away. As this will take 
at least twenty-four hours, the customer may object to waiting 
that length of time. The original maker could, however, duplicate 
the lens while the customer waited, if necessary, and in this way 
it forces the customers to bring their repairs to him. If you desire 
to have a shape of your own, it is well to have a special pattern 
made for this purpose. CHAPTER VI 
Edging 
FOR hand grinding a large stone is preferable, and those 
most commonly used are from 18 to 24 inches diameter by 
Xy-i to 2 inches face. Craigleith, corundum or carborun- 
dum can be used, but the corundum, called by the trade name 
alundum, is the best. This size should run at about 225 revolu- 
tions, or 1415 feet per minute. To obtain the proper speed for 
any size stone, multiply the diameter by 3 1/7, and then multiply 
by the number of revolutions desired. 
The stone should turn away from the grinder and should be 
kept as wet as possible by a drip or sponge on the back. It is 
necessary to have a fair amount of water on the stone, but not 
so much that it will fly all over everything. Keep it wet enough 
so that it will not run dry. With ordinary care the stone will keep 
in shape, but if it becomes rough it should be smoothed with a 
piece of the same material. Do not use a piece of craigleith on 
an alundum, or vice versa. 
The turning can be done by hand, but not as well. When 
turning without a truing device the diamond is held in the right 
hand and the left is used to steady and guide it. A board is 
placed on the trough at the back, so that it just clears the stone; 
this is used as a rest. The diamond tool is then started at either 
edge and rolled along, so to speak, on the rest, so that the stone is 
cut evenly. Enough should be taken off so that the surface is 
square and true. A carborundum block should then be used on 
the back to smooth it, or, in other words, take out the ridges left 
by the diamond. The stone should then be honed with a piece of 
the same material, held in the hands, on the front. 
When the lenses are cut and ready to be ground, they should be 
rinsed in water to remove all the glass dust, otherwise this gets 
into the fingers, and as the lens is revolved, it scratches the sur- 
face. In large shops there is apt to be quite a little waste from 
scratching, and this is usually the cause. A rubber coin pad, 
75 76 
MECHANICAL OPTICS 
such as are often seen on counters, and which can be obtained in 
any rubber store, is convenient to lay lenses on, and prevents 
scratching. 
If the lenses are well shaped with the cribbers, there is very 
little difficulty in putting on the bevel, but if there are bunches 
and points to be ground off, this is where skill is required. The 
shaping is the first important point, and the eye should be trained 
to judge ovals. The axis should also be watched at the same time; 
Figure 71 
see that you do not grind it off. If the lens is cut large, and 
this very often happens, it is a good plan to mark it occasionally. 
In shaping the lens, it should be held between the thumb and 
forefinger of both hands (Figure 71) at a slight angle, turning it 
from left to right. The points and bunches should be taken off 
until you have a good oval. The lens should then be held between 
the thumb of the right hand and the forefinger of the left (Figure 
72). It should then be revolved with the forefinger of the right 
hand as far as possible and then picked up with the thumb of the 
left until you can get another hold with the forefinger of the 
right hand. In this way the lens can be revolved as steadily as 
if it were in a machine. If you have trouble with the lens stick- 
ing to your fingers, a little piece of soap can be kept handy, and 
by just touching it occasionally you will avoid this annoyance. 
The lens should be beveled a little first on one side, then turned 
over and ground the same amount on the other. Too much MECHANICAL OPTICS 
77 
pressure should not be used, as it is better not to try to grind too 
fast. As soon as the lens is beveled on both sides, it should be 
tried in the frame for size. The screw should be taken out and 
the temple removed, and to bring the joints together, an old pair 
of cutting pliers having the edges drilled should be used. 
As the lens is tried in the frame, you should learn to judge 
the amount it will be necessary to grind off. In any case, how- 
ever, the lens should be tried frequently, so that it will not be 
ground too small. It should be fitted so that the joints just come 
together, and no light can be seen between them with the lens 
Figure 72 
perfectly tight, and as the joints are held with the pliers in the 
right hand, try to twist the lens with the left. The bevel should 
be perfectly smooth, and equal on both sides, with no chips or 
bright places; if it does not come out this way the lens was cut 
too small. Just before trying the last time, the lens should be 
held in the right hand between the thumb and forefinger in a 78 
MECHANICAL OPTICS 
vertical position, and allowed to turn on the stone a couple of 
times to take off the sharp edge, otherwise it will chip when the 
joints are screwed up. 
When the lens is placed in the frame see that the strongest 
concave or the weakest convex surface is next the eye, so that the 
axis will be right (the lens should have been cut this way). Also 
see that the ends of the lens come to the center of the joints and 
foot of the bridge. Never mind if the frame is out of shape, put 
the lens in right and true the frame so that the lenses will be hori- 
zontal afterward. In grinding the lens for the other eye care 
should be used to keep the shape the same. 
When grinding torics, coquilles and strong lenses, the frame 
should be curved slightly to conform with the shape of the lens, 
otherwise the eye wire will spring off the bevel, causing the lens 
to drop out. Clerical, or half eyes, are difficult to grind and care 
should be used to fit the corners well before attempting to reduce 
the lens to size. After the corners and top are fitted the surplus 
glass can be ground off the lower part. 
Pebbles cannot be ground successfully on an ordinary alun- 
dum stone, as special grit is required. Carborundum is the best, 
but craigleith answers the purpose very well, although it is slow' 
cutting. 
Window glass should not be ground on your stone, as this has 
a tendency to rough it. 
Strong lenses require a steeper bevel than the regular, and as 
the high power convex are thin on the ends and thick on the sides, 
it is well to start the bevel on the sides first and grind the ends 
last, otherwise the lens will be too full. 
In fitting lenses to rubber, zylonite and shell frames, the frames 
should be softened by using an electrically heated cone like the 
illustration shown in the chapter on Zylonite and Shell Frames. 
When fitting lenses to metal lorgnettes, it is advisable to cover 
the handle with tissue paper or cloth to prevent scratching. It 
is better to send these difficult jobs to the prescription houses, as 
it is often expensive to repair or replace these frames should they 
become damaged or broken. MECZL42VZC4L OPTICS 
79 
Hand Grinding 
For hand grinding the lens is cut y2 mm. larger all around, 
and the first operation is to shape it. It is held in the center 
between the thumb and forefinger of the right hand in a vertical 
position. The left thumb and forefinger are used as a rest and 
guide (Figure 73). The points and bunches are then ground off 
until you have a true oval. The lens is then held in the same 
position, but allowed to revolve slowly. This is done by pressing 
gently against the stone and allowing the lens to slip between the 
fingers in the right hand; the left is simply to steady and guide it. 
Care should be taken not to let it get away from you, but with a 
little practice it can be revolved as steadily as a machine. 
Figtire 73 
After the lens is down to size and the edge is flat and smooth, 
the sharp edge should be taken off, or, in other words, put on a 
very small bevel. Strong lenses and torics will require more bevel 
where the straps are fitted to prevent chipping. In this operation 
the lens is held in the same position as in grinding lenses for frame 80 
MECHANICAL OPTICS 
(Figure 72). If a new pair of lenses is being ground they should 
be measured in millimeters for length and width. The sizes are 
as follows: 1 eye, 37 x 28; 0 eye, x 00 eye, 40 x 31; 
000 eye, 41 x32; 0000 eye, 44x36. Full eyes or short ovals 
are the same length, but 2 mm. wider. 
For measuring, a millimeter rule is considered accurate enough, 
but if you wish to be exact, a Boley gage is better. In matching 
a broken lens, you can lay the new one over the old one and judge 
the size, but this requires practice. The shape should also be 
noted as it is possible to grind two lenses having the same length 
and width, but the shape will be different; one may have full 
corners (Figure 74) and the other pointed (Figure 75). Some 
prefer the shape with full corners, as it gives a straighter surface 
for the strap to bear against, and, consequently, does not loosen 
as easily. 
Figure 74 
Figure 75 
Drop eye lenses (designated by various trade names in differ- 
ent parts of the country) are ground in the same manner. These 
are ground off axis very easily, so this point should be looked out 
for. Also see that the ends are not ground too quickly, as the 
shape will lose its identity and the results will be more like a 
regular oval. 
We have already described the correct methods of grinding, 
but as there are other methods in common use, we will add a 
few suggestions. By correct methods is meant those that are 
most generally used by expert workmen, and are usually adopted 
by the most successful. There are many grinders that prefer 
their own method, and some of these may come easier to certain 
individuals. MECHANICAL OPTICS 
81 
The first method adopted by the beginner for hand-grinding 
is that described and illustrated on page 79 (Figure 73). While 
this is supposed to be used only in shaping the lens, many opti- 
cians continue to grind the entire bevel in this manner. A very 
fair job can be obtained by this method, but the edge will always 
show little points or humps where the position was changed in 
order to obtain a new hold on the lens. For factory work, and 
also prescription work, as it is turned out by the best prescription 
houses, this would hardly answer, as the bevel must be continuous 
and regular in order to pass inspection. 
Figure 76 
The correct and most common method is shown in Figure 76, 
above, but another that is equally good is one where the lens 
is held in the right hand between the thumb and forefinger. This 
position, as far as holding the lens is concerned, is the same as 
i n Figure 76 (is the correct position for rimless grinding), illustrated 
on page 81 of this chapter. Instead of holding it in a vertical posi- 
tion as in grinding rimless lenses, it is held on the stone at an 
angle of 135°. 
If the lens were held in a vertical position, and then tipped to 
one side in order to obtain the correct angle for the bevel, the fore- 
finger would naturally touch the stone. By turning the lens at 
an angle of 135°, a convenient position is found, and at the same 82 
MECHANICAL OPTICS 
time the correct angle for the bevel is obtained. By a little 
experimenting, one can find an angle to suit. When this angle 
has been located, allow the lens to bear on the stone, and it will 
be noted that as the stone revolves, it will have a tendency to 
Figure 77 
carry the lens with it. By pinching the lens a little with the 
thumb and forefinger of the right hand, and guiding it with the 
thumb and forefinger of the left hand, the speed can be regulated 
at will. With this method the lens should be cut fairly close, and 
also have a perfect shape. If the lens is not symmetrical it must 
first be shaped up, using the method before described and illus- 
trated by Figure 73, page 79. As one becomes more expert in 
grinding one will see that it is very essential to have the lenses 
well cut, and cribbed into shape. 
In the last method described, one really handles the lens just 
as though it were in a machine, and no pattern were used. In this 
way the lens follows the original shape, and in grinding we only 
put on a bevel. 
The correct method of rimless grinding by hand is shown in 
Figure 76, page 81. The beginner usually finds it most convenient 
to stand a little to the left of the stone and hold the lens in the 
right hand between the thumb and fore and middle fingers 
(Figure 76). In this method the lens is ground at a point on the MECHANICAL OPTICS 
83 
stone, very near the top. The movement is a long, sweeping 
motion, using the entire arm from the shoulder. First start with 
the lens held so that it touches near the lower end, then move it 
downward in the opposite direction to that in which the stone is 
turning, and roll it, so to speak, as far as possible. This should 
carry the lens to the other end, thereby having ground one side. 
A new hold is taken to turn around the end, then a new start for 
the other side. In this way it requires about four turns to com- 
plete one revolution. 
If there is any shaping to be done it must be looked after at 
this time, and after each sweep of the lens it should be inspected 
for shape. As one becomes experienced, this will require only a 
second or two, and really the grinding can almost be watched on 
the stone. 
After the lens is ground to the approximate size by this method, 
the shape having been found to be correct, the edge will probably 
be more or less uneven. By this is meant that there will be a 
number of ridges at different points where a new start was made 
when changing the position of the lens in the hand. These should 
be taken out, and the lens given a finish that will appear as near 
like a machine-ground lens as we can. This is possible by holding 
the lens in such a position as will allow the stone to cause it to 
revolve between the fingers. The correct way is the same as 
illustrated in Figure 76, page 81, but some of the older grinders 
prefer a method as illustrated in Figure 77. 
In this method the lens is held by the two fingers of both hands 
in such a manner that they act the same as the two shafts in an 
automatic machine. The thumb of one hand, or both if preferred, 
is placed on the edge to act as a stop or governor. A certain 
amount of pressure must be used on the surface of the glass to 
keep the glass from slipping, and if the thumb is not used to 
govern it, it will revolve too rapidly. The revolution must be 
as regular as possible in order that the shape may be preserved 
as well as to produce a perfectly flat edge. As the lens is ground 
in this manner the size should be kept in mind, so that it will not 
be ground too small. When the lens is just to size, the sharp edges 
should be run off as usual, and as heretofore described. 84 
MECHANICAL OPTICS 
Grinding Frameless Lenses by Machine 
After the lenses are in position start the machine and let the 
lenses down to the stone. 
For machine the lens should be cut a little larger than for hand 
grinding; usually about 1 mm. all around is allowed. The marked 
places on the lenses for cutting are used to center the lenses in 
the machine. The lenses are placed in the centering device with 
the cutting lines together, with a piece of rubber or wet cardboard 
between them to keep them from slipping. Extreme care must 
be used to see that these are placed on the pads just right so that 
the axes and centers will be perfect. A little slip at this point 
will spoil your lenses. 
The two principal styles of centering devices now used are 
somewhat similar in the method of using, but radically different 
in construction. One such device is constructed on the clamp 
idea, and is forced open by a spiral spring. The machines fitted 
with this device are supplied with a thick pad resembling a pat- 
tern or former (Figure 78), slotted on the back to engage the 
pins on the head spindle of the machine. The top is oval in shape, 
fitted with a rubber pad in the center and at each end is a hole 
to engage the pins of the centering device. 
Figure 78 
If the lenses have been properly marked before cutting there 
will be no difficulty in placing them on the pad to obtain the 
correct axis and center. If not, it will be noted at this point how 
essential it is to have a conspicuous line on the lens. The center- 
ing device is held in the right hand, and the lens in the left. If 
two or more are to be inserted rubber washers or wet cardboard M ECHA NIC A L OP TICS 
85 
pads should be placed between them. It will also be found more 
convenient to place them together, or build them up in the hands 
first, and then place them on the pad at one time. 
When placing them on the pad, see that the line passes directly 
over the holes in the pad, and that the center comes directly in 
the center of the hole in the middle of the pad also. The centering 
device is then pressed together with the thumb of the right hand 
and held in position with the left hand while the set screw is 
tightened. The lenses should then be held securely, and it will be 
possible to inspect them, and see if they have been placed in 
position correctly. On the top of the prongs of the centering 
device will be found two holes, through which the line on the lens 
can be distinguished, and by sighting through these the line 
should be seen directly in the center. A little study on the part 
of the operator will readily convince him that if the lens has been 
placed on the pad accurately it cannot possibly grind off axis or 
center, provided that it does not slip in the machine. If the lens 
has been placed in position carelessly it cannot grind correctly 
and, therefore, do not blame the machine for any inaccuracy. 
The pattern can be selected and placed on the head spindle, as 
previously described, and the gage set for the size, which is 
governed by the length of lens to be ground. 
For size subtract the width from the length and the difference 
will indicate the number of the pattern to be used. This is placed 
in the machine and the gage set to the length required. For 
example, to grind 40x31, the difference is 9. Fit this pattern 
and set the pointer to 40 mm. 
When the lens has been secured in the centering device, it is 
ready to place in the machine. Open the tail spindle with the 
hand-wheel so that there will be plenty of room to place the lens 
in position. Be particular regarding this point, for if for any 
reason the lens should strike any part of the machine when being 
inserted, it will be very liable to slip, thereby throwing it off axis, 
or out of center. 
Place the holes in the pad over the pins on which the pattern 
or former has been placed, and when set correctly, turn the hand- 
wheel up until the right hand pad strikes the surface of the lens. 
At this point the hand-wheel will turn very easily, or in other 
words, loosely, so that it will appear to slip. It can then be given 86 
MECHANICAL OPTICS 
a turn or two more, very lightly, until the hand-wheel just touches 
the spring on the end of the tail spindle. Do not, under any 
circumstances, turn it up hard, for it will not only do no good, 
but it may damage the machine, by forcing the spring out of 
place. 
When the hand-wheel has been turned up in this manner the 
lens is secured just as firmly as it can be, and if for any reason it 
should slip, it will be because the rubber pads do not hold it 
properly. Either the rubber has lost its elasticity, or else they 
may have become covered with grease. In this case clean and 
dry them, or fit new ones. In fitting new pads, secure them with 
sealing wax, as this is the best method. 
The lens should then be let down to the stone gently and 
allowed to grind around once with a slight spring tension, to 
remove any sharp points. After one revolution has been made 
turn the left hand-wheel, which regulates the spring tension, 
until a firm, even tension is obtained. A quarter turn will be 
sufficient, although this will be regulated by the number of lenses 
to be ground. A single lens will require only a very slight amount 
of spring tension, whereas two pairs can be forced a little more. 
The amount of spring tension used will regulate the speed of 
grinding, although the speed of the stone must also be correct. 
Another centering device is constructed similar to a pair of 
tongs. This is held in the left hand, and opened by squeezing 
the handle together. In this type of machine the pattern or 
former answers also for the pad, having a rubber washer cemented 
on the face (Figure 79). This pad also has holes to engage the 
Figure 79 MECHANICAL OPTICS 
87 
pins on the centering device, and the first operation is to select 
the correct pattern, and place it in position in the centering 
device. The lens is then laid on this pad with the axis and center 
in the correct position, as before described in the other model. 
By releasing the pressure on the handle in the left hand, the lens 
is secured. This type of machine is opened by pulling the hand 
lever towards you a certain amount, to allow the lens to be 
inserted without striking. The spring is then hooked up by a 
chain, the links being so arranged that each one increases the 
spring tension. All other directions apply to this machine as well, 
except the size gage, which is set by pointer instead of a microm- 
eter arrangement. 
Remember that in operating a machine one point to keep in 
mind is to use plenty of water and do not allow the stone to run 
dry. 
The tension should also be regulated according to the number 
of lenses grinding at a time. If too much is used for a single lens 
it will chip. The time necessary to grind a pair of lenses will vary 
from four to ten minutes, depending on the thickness and the 
amount allowed for grinding. Plenty of time should be given, 
however, because if they are removed before they are finished, the 
shape will be irregular. You can easily tell when they are finished 
by the sound. It is unnecessary to remove them immediately, 
however, as they can grind no smaller than the gage. After they 
are ground to size they must be removed and the sharp edges 
taken off the same as if ground by hand. 
When the lenses are removed from the machine, they should 
be wiped immediately, or the grindstone grit is apt to dry on 
the surface, and when cleaned off it leaves marks resembling 
scratches. These are very difficult to remove, but they will come 
off with a wet cloth and fine pulverized pumice. 
Lenses can be edged in the machine without cutting if desired, 
but this is of no advantage unless grinding for stock. Some 
opticians prefer to use their machines this way rather than allow 
them to stand idle, but there is no advantage in it, as interchange- 
able lenses can be obtained of the jobbers at about the same cost. 
The greatest difficulty experienced in grinding by machine is 
the slipping of the lenses. This is caused by the pads becoming 
soft and greasy. It is a good idea when through with the machine 88 
MECHA NIC A L OP TICS 
for the day to place a piece of blotting paper between them to 
absorb the moisture and prevent sticking. 
When placing concave lenses in the machine make sure that 
the pads between the lenses are thick enough, otherwise the 
edges will touch, causing them to crack. 
If flat places or lines, called facets, are found on the edge, the 
stone is out of true and should be turned immediately. 
Some prefer to use the frameless machine without a pattern, 
and when grinding this way the lenses must be watched to see 
that they are not ground too small. This method is perhaps 
quicker, but the machine requires constant attention. 
In cutting, twice as much should be allowed on the ends as on 
the sides (unless your machine has a compensating device). For 
example, to grind 40 x 31, the lens should be cut 41 x 
Provided you do grind nearly all your lenses by pattern, it is 
often convenient to match odd or special shapes by this method. CHAPTER VII 
Drilling 
GLASS can be drilled in various ways, either with a hand 
drill or with power; with steel point or diamond; in an 
upright drill or in a lathe. For drilling, a diamond is 
recommended, which can be obtained from any optical supply 
house. By using care one need never break any diamond. 
A diamond drill should run at from 1600 to 2200 revolutions 
per minute, but 1800 is recommended. If it runs too fast, it 
throws the lubricant out of the hole, and consequently runs dry. 
If it runs too slow, too much pressure is required. These two 
faults are really the cause of all breakage. 
Before starting to drill, see that the stop is set so that the drill 
point just clears the pin on which the lens is placed. There should 
be just space enough to slip a card easily under the drill point. 
The gage should also be set so that the hole will be the right 
distance from the edge. For regular flat lenses this distance is 
.093. Strong convex lenses will be drilled farther in, and strong 
concave lenses nearer the edge. 
All lenses should be drilled, however, for a snug fit and then 
broached or filed the least bit for the variations of the strap. The 
most accurate way is to try the strap on the lens and mark it in 
the screw hole with ink or, better still, with a sharp steel point. 
(This can be made from an old rat-tail file.) In most of the large 
shops the drills are arranged so that the pressure is applied by 
foot power, thereby allowing the use of both hands. This can 
be arranged very easily by drilling a hole through the bench and 
attaching a wire to a foot pedal on the floor. After a little prac- 
tice it will be found that the foot is just as sensitive as the hand. 
There are a great many drilling fluids on the market, and, as 
these are cheap, we would recommend using one of them rather 
than going to the trouble of making it. The theory of a lubricant 
is to keep the drill cool, and in lapidaries water is used on diamond 
drills for drilling all kinds of stones. Turpentine, however, is 
supposed to soften glass, and for this reason it is used principally 
89 90 
MECHANICAL OPTICS 
in making these preparations; "3 in 1 Oil" is also used very suc- 
cessfully by many opticians. 
If you have an up-to-date drill, with a centering device, you 
will set the gage and place the lens in position between the four 
pins, apply the lubricant and place a slight pressure on the handle. 
Do not drill right straight down, however, but raise the drill 
constantly to allow the lubricant to flow into the hole. If it runs 
dry it will show a white powder and more fluid must be applied. 
This is the most particular point in drilling and must be watched 
constantly. The breaking of a drill is usually due to carelessness. 
If you find that the lens has a tendency to vibrate, the point 
of the drill is out of true and should be attended to immediately. 
This is due to the breaking off of a small part of the stone, making 
the point out of center. This can be sharpened, however, pro- 
vided the point is not too short. When buying a new drill, be 
sure that the setting fits the hole in the spindle perfectly. If it is 
small the set-screw will throw it out of center. 
After the lens is drilled half-way through, turn it over and 
drill from the other side. If it is drilled above center it will be 
necessary to change the gage governing the center device to the 
opposite side. 
Tories cannot be drilled in a regular centering device unless it 
has a tipping table or other attachment to tilt the lens. If a drill 
is used without an attachment of this kind, the centering device 
should be removed and the lens held in the hand. The lens should 
be tilted up when drilling the concave side, and down on the 
convex. 
Lenses can be drilled just as well without a centering device, 
but it is necessary to dot the lens. This can be done by the eye, 
or a separate centering device for marking can be obtained for a 
small sum. 
A steel point will drill just as good a hole as a diamond, but 
it is necessary to sharpen it after drilling every few holes. A 
good point can be made from an old rat-tail file sharpened to a 
long point, having two rounded sides. In grinding, however, be 
careful not to draw the temper. A steel drill should run much 
slower than a diamond, or from 600 to 800 revolutions per minute. 
The centering device should not be used, but the lens should be 
held in the hand, and, instead of being held rigidly, it should be 
rocked slightly to allow the drill to cut. MECHANICAL OPTICS 
91 
After you have drilled both sides of the lens so that the holes 
meet, it should be broached out. For this purpose a regular four- 
sided steel broach is used, and is fitted in one end of the idler 
shaft. These will have to be replaced occasionally, as they become 
dull and break the lenses. Some drills are supplied with a broach, 
similar to a rat-tail file, but these are used in the same way. 
Hand drills are not as satisfactory as power drills, but are 
used successfully, however, by opticians wishing to drill occa- 
sionally and who have no power. We recommend running these 
by a foot-wheel, as better results can be obtained. The great 
difficulty with these drills is that you cannot get speed enough, 
and although a slow speed will do for a steel drill, it does not 
work well with a diamond. When using them, however, more 
time must be allowed for drilling, as it is not well to force it. 
We frequently hear of cases where opticians require from three 
to five minutes to drill a hole. In cases of this kind there is some- 
thing wrong; either the speed is not right or else the diamond 
needs sharpening. If a drill is working right a hole can be drilled 
in five to ten seconds. 
Opticians often make the mistake of using a very long diamond 
drill. This is not necessary, and such long drills are very liable 
to break. It is much better to use a short one for all ordinary 
lenses and have an extra one for thick ones. Do not forget that 
diamonds are not guaranteed against breakage, and if you get one 
that is not right, exchange it at once. 
Although the drilling of glass is very simple, many opticians 
seem to experience quite a little difficulty in this work. The 
trouble may occasionally be with the workman, but more often 
it is the fault of his tools. To do good work and prevent break- 
age one should have a good drill. This not only means the dia- 
mond or whatever kind of a point he may use, but the machine 
itself should be as good as can be bought. 
Drilling Machines 
All the best drills are fitted with centering devices and this 
feature alone not only assures one of accurate work, but saves a 
great amount of time. We are illustrating a few of those in com- 
mon use to show the method of using. 
Figure 80 is the Universal and is fitted with a tilting table for 92 
MECHANICAL OPTICS 
the drilling of torics. To drill a pair of Hat lenses it is only neces- 
sary to open the centering device with the left hand and place 
the lens in position between the four pins. Be sure the lens is 
placed so that it touches all four, for if it does not the lens is not 
Figure 80 
in straight, consequently the hole will be drilled off center. To 
guard against this it is well to open and shut the centering device 
once or twice, so the lens may be jarred into position, so to speak. 
When it is secured between the four pins push the centering device 
to right with the left hand, until the edge of the lens comes in 
contact with the gage. This should have been set at the correct 
distance previously. 
With this machine is a small steel washer with several holes 
drilled at different distances from the edge, one side being for 
plus lenses and the other for minus lenses. These holes are num- 
bered to correspond with those on the gage of the drill. There 
is also a small pin furnished to insert in the holes in the strap, 
rhe idea of this device is that the holes in the strap can be 
measured and the gage set at the proper distance. Most opti- 
cians, however, set their drills at the standard distance, which 
is .093 and for new work this will be correct, and on the old work 
the strap can be measured or judged as you prefer. Straps are 
made so accurate at the present time that very little variation will 
be found, and the old ones are usually so much longer that they 
can be detected at a glance and the necessary allowance made 
for them. ME CHA NIC A L OP T1CS 
93 
When the gage has been set correctly and the lens pushed up 
against it, you are ready to drill. Right here it may be well to 
add a few words of caution in regard to the placing of the lens in 
position. In the first place be sure that there is a proper bevel 
on the edge of the lens, especially a strong concave. By this we 
do not mean that the lens must necessarily have a very conspic- 
uous bevel, but on the other hand it must not have a sharp edge. 
If the edge is too sharp it will chip very easily and if perhaps it 
does not chip when placed between the pins it certainly will when 
pushed against the gage. 
Another point that must be looked after is, when a convex lens 
is being placed in position, care must be used to see that the thin 
edge is not pushed under the gage. This sometimes happens 
and even if it is not chipped, it will make the hole come too far 
from the edge. Care should also be used to see that every lens 
lies flat on the table and that one side is not tipped up, for when 
the drill strikes the surface of the lens it will naturally have a 
tendency to force it into position, and this also is very apt to 
cause chipping. Be sure that the lens lies flat on the little pad 
directly under the drill, as this is most essential and the fault of 
many of the old-style drills was that they did not have the correct 
bearing directly beneath the drill point. 
If the lens is to be drilled on center it is only necessary to apply 
the pressure with the right hand, lubricate the point with drilling 
fluid and drill half-way through. Then turn the lens over and drill 
the opposite side. 
At this time we will add a few instructions regarding the sound 
of a drill. Learn to drill by the sound of the point cutting the 
glass. Just as soon as the diamond point comes in contact with 
the surface of the lens a peculiar grinding noise is heard and as 
the pressure is applied and the point sinks into the glass this 
sound can be followed. As the ear is trained you can readily tell 
when the drill is half-way through the lens. The lens is then 
turned over and placed in position as before, but this time be sure 
that the hole is directly under the drill point. If the drill is 
accurate and if you placed it in the correct position in the first 
place there will be no trouble. If some mistake was made, how- 
ever, the holes will not meet and a very poor job will be obtained. 
As the pressure is applied on the second side notice the sound as 
before and when the drill goes through into the first hole it can be 94 
MECHANICAL OPTICS 
detected instantly. In this way a lens is very rarely broken in the 
drilling, as the operator knows just when to release the pressure. 
We have spoken of the sound, and though this is most essential 
we must not forget the hand or foot that applies the pressure. 
As one becomes experienced it will be seen that they seem to 
work together. In other words, it almost appears as if one could 
hear with the hand, as they work so in harmony. 
Finishing the Screw Hole 
When the hole has been drilled through it must be broached 
out, as will be seen by the accompanying illustration. This will 
also give an idea of the correct shape of a drill point. Figure 81 
shows the correct shape of a hole that has been drilled with a 
wedge-shaped point. In this way the hole is tapered and is rarely 
broken in drilling or mounting; also it is much more substantial 
when mounted. When it is run on the broach the center only is 
cleaned out and the hole is left as in Figure 82. A hole drilled 
with a straight drill with a flat point, as in Figure 83, is not only 
very liable to break in drilling, but also after it is mounted. In 
drilling when the two holes meet, the drill goes through with a 
jump, and the shock often breaks the lens. If it does not break 
in all cases, a great many are chipped around the hole. When a 
hole of this kind is drilled and then run on the broach, it is also 
not only liable to break if it is not well lubricated, but it dulls 
the broach very quickly and a dull broach will cause more break- 
age than almost anything else. 
Some of the interchangeable lenses ground at the factory often 
have fairly straight holes and these are drilled this way for the 
reason that they make better-looking lenses. A countersunk 
hole, while more practical and always used in prescription work, 
does not make the best-looking one, whereas a straight hole makes 
a neat-looking lens. 
In the prescription shops it is customary to countersink these 
holes a little, thereby taking off the sharp edges. A countersink 
can be made of steel, a rat-tail file will do, and should be ground 
to a blunt, wedge-shaped point. This can be used in a lathe or 
by hand, by twisting it between the fingers. 
In drilling above the center in this machine the lens is placed 
in position between the pins as before. After it is secure, release MECHANICAL OPTICS 
95 
the catch in the front with the right hand and swing the table to 
the notch marked the amount you wish to drill above. Some of 
these are graduated in millimeters and others in inches; they can 
Figure 83 
be had either way. The lens is then drilled half-way through, then 
turned over and the catch turned to the opposite side, thereby 
swinging the table as before. It is perhaps safer and more accu- 
rate, especially for the beginner, to return the table to the central 
position after drilling one side. In this way one can see to the 
centering of the lens much better than when the table is set at 
an angle. 
For torics the lens should be placed in position while the table is 
in the central position and swung into position afterward, other- 
wise it is almost impossible to place the lens accurately with the 
table tilted upwards or downwards. The correct way is to tilt 
the table downwards first, then place the lens in position with 
the table in a Central position. The table can then be twisted the 
desired amount. After drilling this side, tilt the table upwards 
and turn it to the central position for the centering, afterward 
twisting the table to the correct position for the hole to come 
directly under the drill point. 96 
Af£C7L42VZC4L OPTICS 
The A. O. Co. Factory drill (Figure 84) is constructed some- 
what differently, having an arrangement with two bent fingers, 
Figure 84 
if we may thus term them, instead of the four pins. At the left 
is a lever which is operated with the left hand to open the fingers 
to engage and center the lens. Directly beneath this is a cylinder 
which is graduated for the drilling off center. The method of 
using this drill is to set it for the desired amount off center, then 
with the left hand swing the table forward a little, opening the 
arms to engage the lens at the same time. The lens is then in- 
serted and centered and swung back against the post or gage. 
Tories are drilled on this machine by raising and lowering two 
little slides on the front part of the arms. MECHANICAL OPTICS 
97 
There are many good hand drills. We are illustrating (Figure 
85) one of the higher-priced ones and some of these can be oper- 
Figure 85 
ated with a diamond drill if desired. The only difficulty experi- 
enced is that it is very difficult to obtain speed enough. CHAPTER VIII 
Mounting 
MOUNTING, or "setting-up," as it is usually called in the 
prescription shops, is a branch of the mechanical work 
that requires considerable experience. Almost anyone 
can screw a pair of glasses together, but this is not mounting in 
any sense of the word. The principal feature is to be able to fit 
a lens securely, with all the parts bearing properly, and yet so 
there is absolutely no liability of breakage. 
Breakage is not only expensive, but causes more annoyance 
and delay than anything in the business. One doing this part of 
the work must necessarily be conscientious, whether working for 
himself or for someone else. If one has his mind on the work, he 
can easily tell when the screw is inserted whether it is too tight 
or not. The trouble with most workmen is that the screw is 
driven home as if it were going into a piece of wood, rather than 
glass. When a strap is properly fitted it should be possible to 
twist it sideways j ust a little, until the screw is given the last turn 
or until it is set up. The last turn should make a snug fit. If the 
screw turns hard from the start, breakage is sure to be the result. 
The Bench 
A handy work bench is essential to maximum efficiency. A 
very practical one is shown in Figure 86. As will be noticed it 
provides ample drawer space for tools, etc. The top stands 2 feet 
9 inches from the floor, a convenient height when the workman 
is sitting. It measures 3 feet 3J4 inches long; 2 feet 1 inch wide. 
There is room on top for vise, anvil, soldering or cementing 
apparatus and Calmascope test lenses. One of the features of 
this bench is a wide drawer for files, pliers, screwdrivers, gages, 
etc. Beneath this is a lap drawer which can be left out while 
working to catch filings or small screws that may be dropped. 
98 MECHANICAL OPTICS 
99 
Supplies, tools and materials are comfortably accommodated 
in the three shallow and one deep drawer at the right. A hard- 
wood bench-block protrudes at the workman's left. 
Figure 86 
In order to do good work one must have good tools. There 
are many grades of pliers on the market today, and where for- 
merly any pair that could be obtained in a hardware store an- 
swered the purpose we are now using tools that are made as good 
as surgical instruments. All optical supply houses carry regular 
optical pliers in stock, and although the cost may be slightly more, 
they will last for years. These are well tempered and the round 
and snipe nose can be tapered to very fine points and still stand 
hard work. 100 
ME CHA NICAL OP TICS 
The No. 35 strap plier is the most essential (Figure 87). One 
pair will answer the purpose, but the best workmen usually have 
Figtire 87 
two pairs, with different lengths of lips. Although these are made 
in one length, it is a very easy matter to grind off the lip slightly 
on an emery wheel, and then smooth it with an emery or buff 
Figure 88 
stick. In this way one can have a very short one for the jobs 
that require very little fitting, and yet should be eased a little to 
prevent the strap bearing too tightly on the edges. The one with 
Figtire 89 
the longer lip can be used where the strap must be made con- 
siderably thinner. These pliers can be ordered from the jobber in 
this way if desired. MECHANICAL OPTICS 
101 
A plier known as No. 45 crimpling plier (Figure 88) has proven 
of great value. This tool is not only found very convenient in 
all shops, but it prevents many lenses from being discarded. In 
Figure 90. Rimless strap in position to be reformed. 
Figure 91. Note difference in shape of straps-left one 
as formed at factory, right one showing reforming done 
by pliers. 
Figure 92. Jaws of pliers closed. Narrow straps, curves 
it to fit toric lens and forms relief or offset to avoid break- 
age by binding on concave edge. While held in this posi- 
tion the shank can be bent to any desired angle close to 
strap. 
drilling lenses it quite frequently happens that the hole is too 
near the edge, and also many times old lenses will be poorly 
drilled. When fitting a new mounting it is necessary to shorten 
the strap to fit it perfectly. This plier accomplishes the desired 102 
MECHANICAL OPTICS 
effect at one operation, and in such a way that it cannot be 
detected. 
The other necessity is the snipe-nose (Figure 89). Round and 
flat noses are convenient for some operations, but can be dis- 
pensed with. The snipe-nose should be tapered almost to a point, 
and the inside of the jaws should be perfectly smooth and pol- 
ished. The sharp edges should also be removed. This can be 
done with a fine emery stick with very little trouble. Under no 
conditions should a corrugated jaw be used, as this marks every- 
thing the jaws are used on. 
Figure 93. Detail of rimless strap after being reformed. 
Note relief or offset in strap on concave side. 
Among the newer tools for opticians' use are the Steadfast 
rimless strap pliers, the operation of which is clearly shown in 
the four illustrations, Figures 90 to 93. 
Figure 94 
We should advise all pliers to be purchased in nickel, as they 
not only look better, but are much smoother to handle. A medium 
size is also to be preferred, although many opticians are using 
the larger sizes. These are all right for bridge bending, especially MECHANICAL OPTICS 
103 
for workmen with large hands, but as frameless straps are much 
more easily bent than bridges, the smaller ones are more con- 
venient. 
In mounting lenses speed is quite essential, especially if one 
attempts to do all his own work, as well as refracting. For this 
Figure 95 
reason we are touching on some small points which may seem 
trifling, but it is just the knack of handling tools that makes one 
workmafi excel over others. In the large prescription shops the 
man who has the proper tools and knows how to use them will 
easily be noticed by the way he handles them. 
Cutting pliers are made in many styles. The older styles 
(Figures 94 and 95) are just as satisfactory, as long as they are 
kept in good condition. The trouble in most shops, however, is 
Figure 96 
that one pair of pliers is used for everything. To do good work 
one should have a pair to cut glass screws only, and keep it for 
that purpose. For cutting heavy steel wire and for the odd jobs, a 
heavier pair should be used. The Chappel cutting plier (Figure 
96) is used extensively, as this cuts close to the strap, and requires 104 
MECHANICAL OPTICS 
no finishing. This plier is so constructed that the edges do not 
come together, and in this way they keep their cutting edge for 
a long time. 
There are also several styles of spring cutters (Figure 97) on 
the market, and the advantage of these is that the screw is not 
Figure 97 
cut with a snap. According to theory, a lens, if mounted properly, 
will not break when the screw is cut, but many of them cut with 
such a snap that the jar would naturally cause some breakage. 
Others have a tendency to crawl towards the strap, if the screw 
is cut too close, and thereby seem to pull the screws slightly. 
Whatever plier is used, however, must be in good condition to do 
good work. 
Rat-tail files (Figure 98) should be of the best quality, and well 
tapered. Some are quite blunt, and cannot be inserted in the 
hole, except at the top. A good file should be fairly small and 
Figure 98 
Figure 99 
should protrude through the lens about an inch. The cut is 
optional, but should not be too coarse. No. 0 is quite coarse, No. 
1 medium, and No. 2 is as fine as can be used and yet cut fast 
enough. If a coarse file is used care must be taken to prevent 
chipping. 
A glass screw tap (Figure 99) will be needed, not only to ease 
the thread in the stud, but also to insert through the straps and MECHANICAL OPTICS 
105 
lens for a preliminary trial. It is well to have two of these, one to 
be kept for tapping the strap, and the other can be an old one; in 
fact, one that has the thread worn considerably is to be preferred, 
as it is then inserted easily and quickly. If one has a tap plate 
Figure 100 
Figure 101 
these can be made without much trouble, but as they can be pur- 
chased from the jobber for about twenty-five cents it hardly pays. 
They are made usually from Stubbs's steel. They are threaded 
about one-half inch, and are then flattened on three sides, some- 
what tapering, then hardened. A broach holder (Figure 100) 
or pin vise (Figure 101) will be required to hold the tap, and those 
Figure 102 
with the wooden handle are better for inserting through the strap 
and lens, whereas the regular pin vise is preferable for tapping 
the strap. 
A swivel-top screwdriver is the best for mounting, as in mount- 
ing it is necessary to turn the screw in and out usually several 
times. If a regular wooden-handle spectacle screwdriver is used, 
Figtire 103 
it requires more time, as a new grip must be taken at every half 
turn. In the prescription shops a man using a screwdriver of this 
kind would be so slow that he could never expect to hold his posi- 
tion. We are illustrating several styles of this in common use 
(Figures 102 and 103), the wooden one being quite popular on 106 
MECHANICAL OPTICS 
account of its lightness. These should be used with the swivel in 
the palm of the hand, and not the way a watchmaker usually 
does. Many opticians have been watchmakers and, in fact, are 
doing watchmaking in connection with their optical business 
today; consequently, it is natural for them to get in the habit of 
using a screwdriver with the forefinger on the swivel, twisting the 
barrel between the next finger and the thumb. This in the optical 
business is quite awkward, and the work cannot be done as con- 
veniently or as well. 
If one uses the old style of cutting pliers it will be necessary 
to have a fine flat half-round file to finish the screws. The half- 
round is illustrated (Figure 104), as this is better for bridges where 
the shank interferes. If the Chappel cutters are used, the file 
Figure 104 
will not be needed. Many opticians use the screw-finisher in a 
polishing head, or lathe, and although these produce the best 
finish, it requires a little skill to keep them in shape. This will be 
explained later. 
Figure 105 
There are many other tools that are in use that can be added 
to this equipment, if they suit the individual taste. Among these 
are the No. 39 plier for angling the straps of studs or temples; 
the cushion pad, on which the lens can be placed when inserting 
the screw, and hand broach, a hand countersink; also a hand tap- 
ping machine. A very convenient thing is a large-size tap with 
a quantity of large glass screws for repairing old straps where 
the thread has been stripped, and an Ajax wrench (Figure 105) 
with screws and nuts for Ajax straps. These nuts are sometimes 
used when the thread in the strap has been stripped, and also as MECHANICAL OPTICS 
107 
locks for the glass screws. Buff sticks and wheels are used by 
some opticians to finish the straps, but now this is usually dis- 
pensed with. 
Fioure 106 Group of Wellsworth Optician's tools: M164, Fits U-frame pliers; M170a, shank adjusting pliers; M171, flat-nose pliers; 
FM172, round-nosePpliers; M 172*4, shank bending pliers; M173, pointed-nose pliers; M174, hollow chop pliers; M175, end cutting pliers. 
M176, side cutting pliers; M177, pliers for shaping rimless straps; M177}4, shank angling pliers. 108 
MECHANICAL OPTICS 
It is well to have a good assortment of screws. This should 
include gold, gold-filled and German silver, in regular length and 
extra long, also Ajax and the large size,l'as these are used on 
Figure 107. Another group of Wellsworth Optician's tools: Ml 179, pliers for crumbing lenses; Ml 179a, interchangeable jaws for Ml 179 
pliers; M180, tongs for crumbing edges of lenses; M181, self-opening pliers, parallel jaws, flat nose; M182, self-opening pliers, parallel jaws, 
narrow-pointed nose; M183, self-opening pliers, parallel jaws, round nose; M184, self-opening pliers, parallel jaws, hollow chop, three sizes; 
M186, finger-piece assembling tool; M190, screwdriver, specially designed for use on Windsor frames; M190a. bits only for M190; M192, 
screwdriver, rosewood handle (made in three sizes); M193, screwdriver, hard-rubber handle, revolving lop; M193a, bits only for M193 screw- 
driver; M194, spanners for Ajax washers. MECHANICAL OPTICS 
109 
repair jobs almost entirely. When fitting new lenses to old gold 
mountings the optician can hardly afford gold screws, and the 
filled answer the purpose just as well. A complete assortment of 
screws, in glass bottles, can be obtained in a neat little case 
Figure 108 
(Figure 108). These are inexpensive and are not only convenient, 
but save considerable waste. When a stock of screws are carried 
in open boxes or envelopes they are sure to get mixed, and often 
upset and lost. 
Before trying the lens in the strap it will be necessary to remove 
the screws from the mountings, and as these are inserted at the 
factory by machine, they will probably work hard. The screw 
holes should then be tapped out so that the screws enter easily. 
Trying the Strap 
After the holes in the lens are drilled and broached and the 
studs tapped, the next operation is to try the strap on the lens. 
If the lens is thinner than the strap it will fit into the bottom of 
the strap. Notice whether the parts of the strap that bear on the 
edge conform with the curve of the lens. If not, shape them 
with the snipe-nose pliers. Then take an old tap or a pin about 
the same size and insert it in the screw hole so that it passes 
through the glass into the screw hole of the opposite strap. If 
the lens has been drilled properly, it will be just a little too tight. 110 
MECHANICAL OPTICS 
Now remove the glass and file the hole just a grain toward the 
edge of the glass with a rat-tail file lubricated with drilling fluid. 
Insert the lens again and try a second time. If an old tap is used 
Figure 109 
it acts the same as a screw, so that when it is screwed into the 
hole you can easily judge whether it is too tight or not. If it 
appears the least bit tight do not try to force it, but remove it and 
file the hole a little more. For this reason a small metal screw- 
driver, with a swivel top, is recommended, rather than a wooden 
handle one, as you do not get such a good purchase on the screw. 
You can then tell readily if it binds. One of the greatest mistakes 
is made in forcing screws in the mountings, as this is responsible 
for almost all the breakage. 
When the tap enters the hole easily and there is no play, the 
screw can be inserted, and as it is turned in try the strap and see 
Figure 110 
Figtire 111 
if it is tight. When a strap is properly fitted it should be possible 
to twist it sideways just a little until the last turn of the screw, 
or until it is set up. The last turn should make the snug fit. If 
the screw turns hard from the start, the hole in the glass should 
be filed a little more. 
We have now taken it for granted that the lens and strap were 
just the right thickness (Figure 109). If the strap is too narrow 
the lens, perhaps, will go in but half way. We then take the 
strap plier and insert the plain jaw in the strap and the jaw with 
the hook on the outside (Figure 110). With a slight pressure MECHANICAL OPTICS 
111 
the strap will be widened a little, and if the lens is still tight repeat 
the operation on the other side of the strap. This operation was 
formerly done with a snipe-nose plier before the invention of the 
strap plier. The great difficulty experienced with this tool was 
that the straps were thrown out of alignment and it was necessary 
to square them up and tap them over again. With the strap plier 
this is not only unnecessary, but the work can be done much more 
quickly. 
If the lens is thinner than the strap the plier is inserted with 
the hook on the inside and the plain jaw on the outside (Figure 
111). This will make a slight kink in the strap, very close to the 
lens-bearing parts, which is hardly noticeable. In selecting straps, 
however, it is better to use one a little snug and open it with 
Figure 112 
the strap pliers than to use one that has to be made much nar- 
rower. A convex lens is, of course, thicker at the screw hole than 
at the edge, and for this reason the strap will be made narrower 
at the bottom (Figure 112), otherwise it will twist sideways. A 
concave lens is just the opposite, and the strap should be made 
narrower at the screw holes with a snipe-nose plier. The lens 
should be inserted on the side at the thinnest part and slipped 
along to the screw hole. In fitting strong lenses, especially con- 
cave, see that there is a good bevel on the edge near the screw 
hole to prevent flanking. 
If the hole has been drilled too near the edge it will be necessary 
to bend down the lens-bearing parts and a little variation does 
not matter materially, but if there is any space to speak of these 
ears should be bent with a slight kink near the post (Figure 113). 
This can be done with the strap pliers, but care should be used 
not to take up too much. Under no circumstances bend the points 
down, leaving an opening between the lens and the strap (Figure 
114), as this will work loose almost immediately. In some cases 
where the strap has been opened as much as it will stand in fitting 112 
MECHANICAL OPTICS 
strong concave lenses, or if the workman does not have a pair of 
strap pliers, it will be necessary to file the edge of the lens to 
allow it to go into the strap, but this method should not be used 
Figiire 113 
unless absolutely necessary. It is, however, desirable to do this 
on strong concave lenses. 
In fitting torics the strap should be bent with the snipe-nose 
Figure 114 
pliers to conform to the curve of the glass (Figure 115). Although 
this is really no more difficult, most opticians prefer to send the MECHANICAL OPTICS 
113 
mountings to the prescription houses and have these lenses 
mounted, as the lenses are expensive, and if one is broken it takes 
away the profit. Occasionally an old pair of lenses is sent in to 
be fitted with new mountings and the holes are so large that it is 
impossible to fit a strap so that it will look well. In cases of this 
kind a plug of tinfoil or of wood can be inserted in the hole nearest 
the edge of the lens and this will take up the space between the 
lens and the lens-bearing parts of the strap. There are also strap 
tighteners on the market for this purpose. 
There is also a device on the market supposed to prevent 
breakage and this is used when fitting expensive lenses, such as 
fused bifocals. This consists of a rubber tube and a glass screw, 
with a long pin on the end of the threaded part. After the screw 
is set up it should be cut off and finished. If a screw finisher is 
used the end should be left a little long to allow for rounding. 
If it is cut too short the tool will cut into the strap. If you do 
not use a screw finisher, cut the screw fairly close and file the 
Figure 115 
end with a flat file almost down to the strap. A piece of card- 
board or a brass plate having a square place cut out for the strap 
should be used to lay over the glass to prevent the file from 
scratching. There is also a new plier on the market for this work 
which cuts the screw so close that it does not require filing. 
If the optician does his own drilling he can, of course, drill lenses 
to suit himself. There are, however, a great many who order 
lenses drilled and do the mounting themselves. Many of these 
opticians believe that lenses should be drilled so that all that is 
necessary to do is to screw them into the mounting. If lenses 
were drilled this way a great many would fit too loose, and a poor 
job would be the result. The right way is to have them all a little 
snug and then fit them as before described. To do this a com- 
plete set of tools is necessary and care should be used in fitting. 
If the optician would only realize that men doing the mounting 114 
MECHANICAL OPTICS 
in prescription jobs are the most expert optical workmen and 
have spent years learning the trade, there would not be very 
many complaints regarding the drilling. 
The mounting of frameless lenses is a trade in itself and too 
much care cannot be used in selecting the thickness of lenses and 
straps. The strap plier has simplified the mounting of frameless 
lenses wonderfully and some opticians have a wrong idea of this 
tool. It is not only designed to bend a strap from 2 mm. to 4 mm., 
or vice versa, but it should be used in place of the snipe-nose plier 
in fitting almost every lens. In large shops the men mounting 
rimless work have two pairs, one with a short hook for the regu- 
lar jobs and one with a long hook for extreme cases. 
There are opticians who have an idea that all frameless lenses 
should be so accurately drilled that no fitting should be required. 
Although lenses are drilled to a standard gage and straps are 
milled the same, it is impossible to have them fit perfectly. A 
slight variation in the size of the diamond point will change the 
size of the hole; also some lenses are countersunk and others are 
drilled straight, as before explained. If one is in the habit of 
ordering lenses from the prescription houses, unmounted, he 
should expect to do a certain amount of fitting, and in order to do 
this he should have a proper equipment of tools. 
Figure 116 
We have already explained in preceding chapters the method 
of adjusting the strap with strap pliers. Many opticians still use 
the old method of bending the strap with snipe-nose pliers, how- 
ever, and although on the average the work is not so well done, 
we will give an explanation. 
The strap, of course, should be slightly wider than the lens, 
but just enough so the lens will slip in easily. If it is too wide, MECHANICAL OPTICS 
115 
grasp one side of the strap with the point of the pliers in the 
middle and bend it slightly outwards. This will make a slight 
kink near the lens-bearing part (Figure 116). Now bend it back- 
ward to its original position and you will find that it has made 
the opening slightly narrower. By repeating the operation on 
the other side of the strap both sides will be alike and the strap 
will be made uniformly narrower. The strap should then be 
sighted endwise, to see if the sides of the strap are parallel, as in 
bending them with a plier of this kind, they are very apt to be 
Figure 117 
out of alignment. After bending them parallel the screw holes 
should be tapped, and this will also have a tendency to bring 
them in line the other wray. It is very necessary to have the strap 
in as perfect condition as possible, for if it is bent out of line in 
any way breakage is sure to be the result. 
One of the great advantages of the strap pliers is that they 
accomplish this result without altering the original alignment of 
the strap. This operation is for convex lenses and in making the 
inner part of the strap narrower we prevent the lens twisting. 
If this is not properly done, or the lens is fitted in the strap with 
the jaws parallel, it is possible to hold any eyeglass or spectacle 
by grasping both lenses and twist it out of shape. This, in many 
cases, is the cause of lenses flaking near the strap. 
If the strap is too narrow it must be opened to admit the lens 
sufficiently, and yet not bind on the edges. In most cases con- 
cave lenses require this operation and it is very difficult to fit them 
properly, without the strap binding on the edge of the lens, if it 
is not done with a strap plier. In opening the strap grip one side 
as near the bottom as possible; that is, near the lens-bearing part, 
and open the strap outwards as before. Remember, however, 
that for the convex lenses we grip it near the middle, whereas for 
concave we grip it as far back as possible. 116 
MECHANICAL OPTICS 
When the strap has been spread on one side (Figure 117) grip 
it near the middle and bend it back to its original position. It 
will then be noticed that this side has been somewhat rounded 
out. By repeating this operation on the opposite side quite a 
little extra width can be obtained. Sight the strap endwise and 
straighten as before explained. 
Some of the newest style bridges have an invisible foot; in other 
words, the shank is soldered in a depression on one side of the 
strap. In this style it is practically impossible to bend the side 
on which the bridge is soldered and, consequently, all the varia- 
tion must be made on the opposite side. 
In doing this the strap will be shortened somewhat and it will 
probably be necessary to file the hole slightly to prevent the screw 
Figure 118 
binding. If the lens is concave and it is found that it binds slightly 
in the bottom of the strap, it will be well to file the edges slightly, 
to allow for the rounding of the strap. 
When filing a lens lay it on the edge of the bench perfectly flat, 
with the hole towards you. Hold it with the left hand, and with 
a rat-tail file which has been dipped in drilling fluid file aV-shape 
slot from the hole to the edge of the lens. Unless a very fine file 
is used, this will cut very rapidly, so care must be used not to 
get it deeper than is necessary. A little filing on both sides is to 
be preferred, rather than all on one side. Care must also be used 
to see that the point of the file does not cut on the opposite side 
of the hole (Figure 118), as this makes a very bad-looking job. 
In doing this work al'ways use a rat-tail file, as this produces a 
V-shape slot and takes off that part of the glass that binds. It 
must be remembered, however, that in filing a lens it is weakened MECHANICAL OPTICS 
117 
considerably, and with the strap pliers it is very rarely necessary. 
If this must be done, however, take off as little as possible. 
In fitting concave lenses where the strap is wide enough to 
take the lens without filing, the strap can be bent edge-shape and 
the lens inserted on the thin edge, and you can then slide it around 
to the hole. In this way the strap is right down into position and 
requires no bending. When the edge must be filed to take the 
lens it is impossible to do this, consequently the strap must be 
Figure 119 
forced over the edge. A pair of pliers with one long curved jaw 
(Figure 119) is very convenient to press the strap into position 
after the lens has been inserted. 
If the hole has been drilled too near the edge the lens-bearing 
parts must be bent down to conform to the curvature of the lens. 
This operation has been described with the regular pliers, but 
Figiire 120 
the No. 45 crimpling plier accomplishes this result with one 
operation and in such a way that it cannot be detected. This is 
made with a slot in one jaw, which is inserted over one side of 
the strap. It is also shaped to fit the lens-bearing parts. The 118 
MECHANICAL OPTICS 
opposite jaw has an opening which fits around the post, and when 
the pressure is applied, it forces the lens-bearing parts downward, 
thereby shortening it slightly. When the lens is inserted no open- 
ing can be seen, as it is covered by the side of the strap. The 
accompanying illustration (Figure 120) shows a poorly fitted 
strap; also one that has been formed with this plier. It is impos- 
sible to say too much regarding the fitting of straps, as the num- 
ber of poor jobs in use today is almost a disgrace to the profession. 
In the last few years the work has been greatly improved, how- 
ever, and this is especially noticeable when one considers the num- 
ber of straps and other devices that have been placed on the 
market to overcome this defect. 
Ajax straps do not have to be bent in this way. If the side of 
the strap (there is but one in this style) has been bent backwards 
the tendency is to bend the lens-bearing parts down, the same as 
Figure 121 
a regular strap, but this way is incorrect. By bending the side 
upwards a little-in other words, making the angle of the side of 
the strap and the lens-bearing parts more acute, the lens will fit 
properly. If the hole in the lens is just the right distance this 
should be a right angle, but even if it is not, it will make no mate- 
rial difference. 
Some opticians grind off the edge slightly when the hole has 
been drilled too far from the edge. This not only requires extra 
time, but also changes the shape of the lens slightly. It is much 
better and more convenient to file the holes, as it cannot be 
detected and is just as strong. 
In mounting many workmen have their own peculiar ways of 
fitting straps. For example, if the screw binds slightly at the 
start the pressure can be relieved by pressing firmly with the side 
of the screwdriver on the end of the stud or strap. This forces 
the lens-bearing parts back slightly and frequently the screw will 
then turn easily. If there is a slight opening on the sides these 
can be closed up with a slight pressure of the snipe-nose pliers MECHANICAL OPTICS 
119 
on the strap. Pliers for this purpose can be made by grinding 
out the inside, so there is no danger of edge-chipping by the 
jaws of the plier. These little points are referred to only to show 
how it is possible to increase the speed, but they are somewhat 
dangerous for the inexperienced. 
Torics 
Toric are no more difficult to mount than flat lenses, provided 
that the strap is bent as described in our previous articles. All 
other instructions relating to flat lenses apply to this form of lens 
also. Do not, however, attempt to fit a toric lens into a strap 
that has not been shaped to fit. We see so many fitted as in 
Figure 122 
Figure 121 that a word of caution is necessary. By studying this 
drawing for a moment one can easily see the causes of so much 
breakage in mounting these lenses. 
The best method of finishing the ends of screws is with a screw 
finisher. There are two kinds on the market-the upright 
(Figure 122), which resembles the ordinary drill press, and the 
horizontal (Figure 123), which is similar to a buff head. The 120 
MECHANICAL OPTICS 
upright supports the screw a little better, perhaps, but the hori- 
zontal is quicker. In using these machines the screw should be 
cut a little long, and if the ordinary cutting pliers are used the 
bevel on the jaw will leave them about the right length. If the 
close cutters are used see that they are not cut too short. When 
holding the screw up to the cutters place the forefinger or thumb 
against the screw-head to support it somewhat and rock the glass 
slightly to prevent the screw cutting to a point. The condition 
of the knife will also have considerable to do with the shape of 
head. The knife should be honed with a very small oil stone, 
Figure 123 
having a rounded edge, and kept as sharp as possible. When 
inserted in the holder it should be adjusted so that it will cut just 
enough to make a half-round head. 
If the horizontal head is used it must be run left-handed, or in 
other words, from you. All the cutters are made to cut this way 
and many are out of use today that have not given satisfaction, 
for the reason that they have been run the wrong way. It must 
be remembered that buff heads run toward you, so for this reason 
it is necessary to have a separate machine for the screw-finisher. 
The advantage of finishing screws in this way is that it requires 
no polishing and also forms a slight burr on the end of the screw, 
and, according to theory, it should not work loose as quickly. 
Under any condition, take pride in having your work done well 
and in such a manner that if it is taken to a competitor for repairs 
it will bear inspection and cannot be criticised. CHAPTER IX 
A LTHOUGH single-piece and fused bifocals are most largely 
/"A in use today, yet there are many cement bifocals still called 
for and this chapter on bifocals will, therefore, treat first 
of the older type, the cemented, two-piece bifocal. 
The first step in making a pair of cement bifocal lenses is to 
select the stock. The distance lenses will be written on the pre- 
scription and although any form of lens can be used, it will make 
considerable difference in the price of some combinations. It is 
also desirable to place the wafers next the eye, if possible, as it 
makes the reading segments less visible. 
When making a pair of sphericals, periscopic distance lenses 
are selected; first, on account of the better form, and secondly, 
on account of the uniform curvature of the inside surfaces. This 
surface on American ground lenses is always - 1.25 on periscopic 
convex lenses, and -T 1.25 on the outside of periscopic concave 
lenses. With concave lenses it does not make so much difference, 
as we will show later. On account of the uniform inside surface 
of pcx. lenses, the factories adopted a uniform stock of wafers, 
having a + 1.25 surface on one side, and the balance of the power 
on the other. These are known as "regular wafers'' and are 
ordered by this term. They are also the lowest price wafers that 
are manufactured. 
We will take, for example (Figure 124), a prescription distance 
+ 2, reading + 3.50, or it may be written, distance + 2, add + 
1.50 for reading. We select a pair of pcx. lenses + 2 and a pair 
of regular + 1.50 wafers. These wafers will be + 1.25 on one 
side and + .25 on the other, the + 1.25 curve will be cemented, 
of course, to the - 1.25 surface of the distance lenses. After 
these are cemented, it must be remembered, the contact surfaces 
(that is, the - 1.25 and the + 1.25) do not have any effect on 
the power of the reading portion. We simply add the powers of 
the outside surface of the distance lens and the outside surface of 
the wafer, to obtain the reading power. In this prescription the 
Bifocals 
121 122 
MECHANICAL OPTICS 
distance lens being + 2 and having an inside surface of - 1.25, 
must, of course, have an outside surface of + 3.25. The outside 
Figure 124 
Figure 125 
surface of the wafer being + .25, we have the desired + 3.50 
power. Many opticians seem to experience quite a little difficulty 
Figure 126 
Figtire 127 
in figuring these wafers, but if it is remembered that the contact 
surfaces have no effect, except that they must be the same in 
order to fit so they can be cemented, there will be no trouble. 
Take, for example (Figure 125), a prescription with + 2 distance 
and + 2.75 reading. The wafer in this case will be + 1.25 on MECHANICAL OPTICS 
123 
one side and - .50 on the other. This is really a compound wafer, 
but as the factories are obliged to furnish all combinations of 
"regular wafers," you really obtain them at the same price. 
Now suppose we had selected for the first prescription (Figure 
126) a pair of double convex lenses for distance + 2. There 
would have been + 1 on each side, and to fit a wafer we must 
have one with a - 1 surface. This could not be selected from 
the stock of regular wafers, as there would be no such combina- 
tion. As we must have this curve, it must be surface ground, and 
to make the required addition of + 1.50, we must have on the 
other side + 2.50. We then have a combination of - 1 O + 
2.50, and this we call a compound wafer, for the reason that this 
form is used to fit to compound lenses. It can readily be seen 
that the price will be the same as if fitted to compound lenses, 
and although the distance lenses cost no more, the extra expense 
of the wafers makes the job cost more than is necessary. Many 
times a prescription is sent to the prescription houses, either 
ordering double-convex cement bifocals, or else the optician fits 
a pair of double convex lenses to the frame, and sends them in 
to have the wafers fitted, and then is unable to understand the 
extra charge. 
It should always be remembered that cement bifocals in con- 
vex sphericals should always be ordered pcx., or if no specifica- 
tion is made they will be made in this form, unless for some 
reason double convex is wanted, in which case the extra charge 
will be made. As before stated, periscopic concave lenses are all 
made with a + 1.25 outside curve. It does not make a desirable 
form to fit wafers to the outside of these lenses, so all wafers are 
fitted on the inside, consequently they must be ground to order. 
For example, we will take (Figure 127) prescription distance -• 
3.50, reading - 2, the reading lenses of course always being the 
weaker. As the outside curve is + 1.25 we must have on the 
inside - 4.75. To fit this surface we must have a + 4.75 surface 
on the wafer; the other surface will then be - 3.25. The combi- 
nation of the wafer will then be - 3.25 O + 4.75. This is a 
compound wafer, consequently costs extra. 
Now suppose we select for this same prescription (Figure 128) 
a pair of double concave lenses - 3.50. These lenses will be - 
1.75 on each side, and as the power of the wafers is to be + 1.50, 
and one surface must be + 1.75 to fit the - 1.75 surface, we must 124 
MECHANICAL OPTICS 
have a wafer + 1.75 O - .25. This is also a compound wafer 
and must be ground to order, the same as the first form. In this 
case it will be seen that it makes no difference in the expense 
whether they are made in the double concave or periscopic con- 
Figure 128 
Figure 129 
cave form. As this is true of all combinations of concave cement 
bifocals, as well as those having a concave distance and conve 
reading, it makes no difference as far as the expense is concerned 
which way they are ordered. As the periscopic form is to be 
preferred and there is no difference in the expense, it is better 
to make them this way. 
Cylinder cement bifocals require what are known as "piano 
wafers." As cylinders have one side piano the wafers must neces- 
sarily be ground in such a form that one side will be piano and 
these surfaces must always be cemented together. Under no 
circumstances can a wafer be cemented to the cylinder surface 
of a lens, as this will destroy the cylinder effect. Although it is 
generally understood that two surfaces to be cemented must be 
the same curve, we often find surfaces cemented that are not 
the same and occasionally we find wafers cemented to the cylinder 
surfaces. According to theory, this is impossible, and if it is 
possible to make them hold for a while they will usually come 
back in a very short time to be re-cemented. When a job is re- 
ceived by a prescription house, with the complaint that the 
cement does not hold, the surfaces are first inspected to see if they MECHANICAL OPTICS 
125 
are correct, and in many cases poor contacts are the cause. In 
selecting wafers for cylinders no trouble will be experienced, for 
the power is entirely on one side. For example, a + 1.25 piano 
wafer is + 1.25 on one side and piano on the othei. It will be 
noticed, however, that a + 1.25 piano wafer and a + 1.25 regular 
wafer are the same. 
Compounds require special wafers, as has already been stated, 
and although these seem to give more trouble to the optician, 
they are really just as simple. If one point will be remembered, 
it will help to simplify the figuring of these wafers, and that is, 
always write out the distance and reading correction in full, and 
the surfaces will be right before you. For example, take a pre- 
scription distance + 2 O + .50 ax. 90, add + 1.50 for reading. 
Figure 130 
Figure 131 
If we attempt to figure the surfaces of the wafers from this pre- 
scription it is somewhat difficult. If we write the prescription 
(Figure 131): 
Distance + 2 O + .50 ax. 90 
Reading + 3.50 O + .50 ax. 90, 
we already have the inside and outside surfaces figured. You 
will remember that the contact surfaces have no effect, conse- 
quently all we have to do is to select a wafer to fit the + 2 surface 
of the compound, which will be - 2; the other surface will be 
the reading power, which is + 3.50. As the two outside surfaces 
will give the reading power, it will be seen that the outside sur- 126 
MECHANICAL OPTICS 
face of the wafer is + 3.50 and the outside surface of the com- 
pound is + .50 ax. 90, and we have the reading correction, 
+ 3.50 O + .50 ax. 90. This is true of all compounds whether 
+ O +, - O -, + O - or - O +, and to illustrate, take 
for example (Figure 130) prescription distance - 3.50 O + .50 
ax. 90, reading add + 1.50. This prescription will be as follows: 
Distance - 3.50 O + .50 ax. 90 
Reading + 2 O + .50 ax. 90. 
The wafers will have a + 3.50 surface to fit the distance lens, 
and - 2 for the reading power. When cemented the outside of 
the distance lens will be + .50 ax. 90 and the outside of the 
ft * /ao'' fo 
Figure 132 
Figtire 133 
wafers - 2; hence we have the reading correction - 2 O + .50 
ax. 90. In making compound cement bifocals it is customary to 
fit the wafers next the eyes, whenever possible, in order to make 
them inconspicuous. The rule in most prescription houses is to 
place them inside, whenever the reading power is less than + 4.50, 
and when it is stronger it is placed on the outside. It is necessary 
to use your judgment, however, it being poor form to place a 
+ O - compound or any lens having a concave surface, with 
the weaker surface out. 
Tories are somewhat more complicated, unless the prescrip- 
tion is transposed so the spherical surface is inside. If the pre- 
scription is not written in this way it should first be transposed MECHANICAL OPTICS 
127 
and the rest will be simple. For example, take (Figure 131) pre- 
scription distance + 2 O + .50 ax. 90, reading add + 1.50, this 
will be 
Distance + 2 O + .50 ax. 90 
Reading + 3.50 O + .50 ax. 90. 
The distance lens if ground on a + 6 curve would be + 6 O + 
6.50 on the outside and - 4 inside. To fit this surface we must 
have a + 4 curve, as the power of the wafer is + 1.50; we sub- 
tract this from the + 4 and we have - 2.50. By then referring 
to the outside surfaces we find that the distance lens is + 6 O + 
6.50, and the wafer is - 2.50. The difference between the + 6 
and - 2.50 will give the reading power of + 3.50 O + .50 ax. 90. 
In this case it will be seen that the + 6 base curve must always 
be considered, consequently the reading power is always obtained 
by fitting a wafer with the outside surface having a curve of 
such a power that the difference between this surface and the 
base will equal the reading prescription. If this prescription 
(Figure 132) had been written distance + 2.50 O - .50 ax. 0, 
reading add + 1.50, with the request that it be filled as written, 
and not transposed, we would be obliged to fit the wafers on the 
outside. This form of lens would have - 6 0 - 6.50 on the 
inside, and + 8.50 on the outside. 
As we have already explained, a wafer cannot be cemented to 
the cylinder surface, as it will destroy the cylinder effect. The 
outside surface being + 8.50, it will be necessary to have - 8.50 
surface on the wafer to fit, and as the whole power of the wafer 
must be + 1.50, the other surface will be + 10. We then have 
the combination - 8.50 O + 10. It will be seen these are ex- 
treme curves and are not only hard to surface-grind on account 
of the extreme thinness, but will be just as hard to edge and 
cement. A job of this kind is not only more expensive, but as a 
rulea numberof wafers are broken before a perfect job isobtained. 
In making toric bifocals it is always preferable to transpose 
the combinations so a spherical surface can be obtained on the 
inside, even though it is a weak curve. If the spherical of the 
distance lenses is very strong the full inside curve can be obtained 
by using a + 9 D. base. 
All combinations of torics are worked out in the same way and 
if all combinations be transposed to a form having a + cyl- 128 
MECHANICAL OPTICS 
inder there will not alone be less trouble in figuring the wafers, 
but the curves will not be extreme. 
Occasionally we are requested to make a pair of bifocals (Figure 
133) as follows: Distance + 1 O + .50 ax. 90; reading, + 20 
+ 1 ax. 90. This combination has a different power cylinder in 
the reading correction, and although it may be contrary to the 
rule and unnecessary, it can be made by fitting two (2)wafers, 
one on each side. As the contact surfaces do not count, the out- 
side surfaces must be ground to produce the desired effect. We 
must first have a surface - .50 ax: 90 to fit the + .50 ax. 90; on 
the outside we will then grind + 1 ax. 90. 
This makes a wafer with two cylinder surfaces, and this is 
not only difficult to grind, but is very expensive. The other wafer 
w'ill be a regular compound form, having - 1 contact to fit the 
+ 1 sph. surface of the compound, the other will be + 2. We 
then have the reading correction, + 20 + 1 ax. 90. It shows 
that a combination of this kind is obtained by an error in the 
refraction, as astigmatism is constant and does not change for 
reading. 
Cross cylinders cannot be made in bifocals and these com- 
binations should be transposed to compounds and wafers fitted 
in the regular way. 
In making cement bifocals the distance lenses are ground the 
same as usual, except that when grinding to frame they should 
be left a little large, or about 1 mm. between the joints. The 
wafers are about 30 mm. round in the rough, and if the prescrip- 
tion is such that one wafer can be used for two, it should be split 
in the center, using a hand diamond or a sharpened rat-tail file. 
They can, however, be edged in quantity in the frameless machine 
before splitting, but care should be used not to screw them in too 
tightly. It is not practical to attempt to edge less than a pair. 
If ground in this way they will be left 28 mm. diameter and then 
cut in the center. 
It is always advisable to center wafers when possible, and if 
the prescription reads, for example, distance + 2, reading + 4, 
the power of the wafer is equal to the distance. As a spherical 
lens is practically two prisms with the bases together, by fitting 
the wafer with a thin edge upward and the thick downward, the 
The Distance Lenses MECHANICAL OPTICS 
129 
prismatic power is neutralized (Figure 134). When the power of 
the wafer is stronger than the distance, it is necessary to cut it 
nearer the center to obtain less prismatic power, consequently 
two wafers must be used. When the power of the wafers is 
Figure 134 
Figtire 135 
weaker than the distance lens, it will be cut as far from the center 
as possible, to obtain all the full prism power. 
It is, however, practically impossible to vary this very much, 
as the wafers are not large enough. The only way that this can 
be obtained is to grind prism wafers. It is not absolutely neces- 
sary to do this as a little variation in the center does not matter 
much except that the centers must always be on the same hori- 
zontal plane. In other words, do not have one center up and the 
other down-i.e. one wafer centered and the other not-as this will 
cause the patient quite a little discomfort perhaps, and when he 
returns, complaining, you will be at a loss to know the cause of 
the trouble. 
When fitting piano wafers to cylinders with the axis at 90°, 
always cut them from the center (Figure 135). 
After the outside edge is finished (leaving the bottom rough) 
it is ready to be cemented. Clean the distance lens and the con- 
tact surface of the wafer carefully with alcohol. Put a small 
amount of prepared balsam (Figure 136) on the lens and place 
the wafers in position. Hold the lens over an alcohol flame, or on 
an electric stove. An ordinary wood spring clothes pin is very 
convenient for holding the lens; or tweezers can be used. Heat 
the lens gradually until the cement flows freely, then remove it 130 
MECHANICAL OPTICS 
from the heat and allow it to cool; at the same time place the 
wafer in position with a stick or tweezers, being careful to press 
out all When it is cool, see that it is firm and cannot 
be moved. If it appears yellow when laid on white paper, it is 
cooked too much and burned. 
Figure 136 
Strictly speaking, all wafers should be set in 1 mm. toward 
the nasal side, although this is not necessary when using large 
wafers. The smaller styles, such as the horseshoe shape, should 
be cemented in this way. 
Concave Bifocals 
In making concave bifocals the wafers should be cut so that 
the thickest part is upward (Figure 137). This is to neutralize 
the prismatic effect of the distance lens. After the wafer is 
properly placed and the lens is cool (Figure 138), the lower edge 
can be cut off, using a pattern the shape of the distance lens. It 
is then beveled, if for frame, and ground down to size; if for frame- 
less, it is ground to conform to the shape of the distance lens and 
beveled slightly to take off the sharp edge. MECHANICAL OPTICS 
131 
The lens should then be cleaned with alcohol, or naphtha if 
preferred. If it is then found that the cement has started, warm 
the lens slightly and the spots will probably disappear. 
Figure 137 
Figure 138 
Frameless Cement Bifocals 
In making frameless cement bifocals it is a good plan to drill 
the lenses before cleaning, as this prevents the drilling fluid work- 
ing under the wafer. Wafers should always be fitted to frameless 
lenses if possible before drilling, as there is less liability of cracking. 
Miscellaneous Pointers 
In the grinding of a cement bifocal either a composition or 
Craigleith stone can be used, although a Craigleith is preferable, 
not grinding as fast. Wafers should be very thin, and when using 
a fast-cutting stone care must be used to prevent their grinding 
away too fast, and thereby producing a poor shape. As a rule, 
wafers come from the factory with very clean edges, but even 
if they are more or less irregular it is unnecessary to cut them, 
unless, of course, the center is to be used. 
In the majority of cases where the wafers are split in two 
before grinding there is very little difficulty in shaping them up. 
If they are ground separately be particular to see that the shape 
is uniform. The regulation size is 28 mm. diameter and the 
height is varied according to the width of the distance lens, 
although usually from 13 to 14 mm. high. The rule is to set the 132 
MECHANICAL OPTICS 
top of the wafer about 2 mm. below the center of the distance 
lens, although if worn constantly a lower wafer is more satis- 
factory. This depends entirely on the patient and, to a large 
extent, the purpose for which they are to be used. For example, 
for a bookkeeper using them almost constantly, a large wafer 
may be preferred (Figure 139), whereas a person doing very little 
close work and who uses them only occasionally for near work, 
will prefer them very small (Figure 140). 
Figure 139 
Figure 140 
Wafers can be obtained from the supply houses all edged, ready 
to cement, and the regular size is 13 x 27 mm. To obtain this 
size when doing your own grinding it is necessary to grind them 
28 mm. diameter, as before stated, for the reason that, when ce- 
mented to the base lens, the bottom part will be cut off, thereby 
making them smaller. 
The principal difficulty the beginner finds, after he has learned 
to cement, of course, is in placing the wafer. When the lens 
is placed on the bench, after having been heated, or, in other 
words, the cement has boiled, it will require some few seconds to 
cool. During this period there is plenty of time to move it around 
and place it in position. Once your eye has become accustomed 
to the task it will soon be a simple matter to place the wafer with 
fair accuracy, and when one is placed just where you want it you 
will then have a pattern for the second lens. 
If after allowing it to cool somewhat you find when laying one 
over the other they do not match exactly, there is no harm in 
warming one or the other slightly and moving the wrnfer to the 
proper position. It is a very simple matter to get the wafers 
a different size in cementing by placing one higher than the other, MECHANICAL OPTICS 
133 
even though they may have been ground alike. One-half a milli- 
meter difference is very noticeable (see Figure 141). This cannot 
Figure 141 
be done many times, however, as either the cement will be cooked 
too much or else the cement will be squeezed out so it will be 
too thin. In this case bubbles or air spots are very likely to 
appear and it will then be necessary to take the lens apart and 
start all over again. 
When the lens is finished and a bubble or base spot appears 
it is sometimes possible to remove it by warming the lens slightly, 
when it will generally disappear. It should first be tried without 
applying any pressure to the surface of the wafer, and then if this 
does prove successful it will do no harm to press the wafer down 
a little. It must be remembered, however, that a lens should be 
heated but slightly; in other words, just warmed. For this work 
the regular tweezers have always been used, but recently there 
have been improvements made which are quite a help. Figure 
142 shows an improved tweezer, which holds the wafer in position 
while cementing; it does not, however, assist the workman in 
placing the wafer. Figure 143 is a more complicated form, which 
can be termed a gage. This tool is constructed so that the jaws 
Figure 142 
can be spread to allow the insertion of the distance lens. This 
can be set squarely so there is no difficulty in placing the wafer 
in the correct position. By the use of the scale at the bottom the 134 
MECHANICAL OPTICS 
wafer can be centered or decentered accurately. All wafers should 
be decentered in from 1 to mm. to allow for convergence, 
but the large majority are placed in the center. The lever D is 
Figure 143 
used to apply the pressure to the surface of the wafer, to cause 
the bubbles to disappear. 
Electric heaters (Figure 144 and Figure 216, page 219) are now 
used to a great extent in bifocal work, as the heat is more even 
and can be regulated at will. These are inexpensive and can also 
be used for inserting lenses in rubber, zylonite or shell lorgnettes, 
as well as eyeglasses and spectacles. This heater is almost indis- 
pensable in Opifex work. 
Neufocal and Thin Wafer Bifocals 
There is a form of cement bifocal which has a wafer ground 
extremely thin by a special process. These are ground on a thick 
"backing," so that they can be edged as easily as any lens or, if MECHANICAL OPTICS 
135 
preferred, they are supplied edged. Figure 145 illustrates one of 
these wafers in the rough, although the edge is nearly perfect, 
being as sharp as a knife edge and perfectly round. 
Figure 144 
These wafers can be edged by hand fairly well, but the shape 
must be looked after very carefully. In grinding it is only neces- 
sary to grind away the backing and simply touch the edge of the 
wafer as slightly as possible. This will clean off any trace of 
Figure 145 
Figure 146 
Figure 147 
rouge and at the same time take out any minute chips. Opticians 
who have edging machines will find it to their advantage to have 
special small pads made to accommodate these wafers, and by 
grinding them by machine a great deal of time will be saved, and 
also insure a perfectly ground wafer. Figure 146 shows the wafer 
edged and ready to cement. 
Before cementing it will be necessary to ascertain which surface 
is cemented to the backing. A lens measure, constructed espe- 
cially for measuring wafers, is essential to measure these curves 
accurately (Figure 148). In some cases the contact surface will 
be upward, and in others it will be cemented to the backing. For 136 
MECHANICAL OPTICS 
this reason it requires two different methods of handling. If the 
contact surface is up it should be cleaned and placed on the heater 
to warm it slightly. As this requires but a few seconds the base 
lens can be cleaned and placed on the heater at the same time. 
With a small stick or a pair of tweezers the wafer can be pushed 
Figure 148 
off the backing slightly (Figure 147) and after the cement has 
been applied to the base lens, grasp the backing and turn it over 
so that the wafer will be downwards and apply it to the base lens 
(Figure 149). By then holding the wafer with the stick the back- 
ing can be removed and the wafer then placed in position. With 
the electric heater this operation can be done entirely on the iron 
plate, but if an alcohol lamp is used care should be taken not to 
heat the .wafer too much or it will crack. 
If the contact surface is cemented to the backing it is unneces- 
sary to clean it, but simply start the wafer from the block, as 
before described, and transfer it to the base lens without turning 
it over. If the wafer has been surface-ground in this form it is 
possible to bevel the edge slightly before transferring it, but this 
is not really necessary. It must not be done if the contact surface 
is upward, however, as the bevel will be on the wrong side. If for 
any reason the contact surfaces should show any specks of dirt 
after they have been cemented, the wafer can be transferred to MECHANICAL OPTICS 
137 
another lens, or backing, cementing it, of course, in the regular 
way. It does not matter if the surfaces do not fit, as the 
cement will have body enough to allow the contact surface to 
Figure 149 
be cleaned. In recementing old lenses it is necessary to transfer 
them in this manner, to clean off the old cement. Be sure to 
heat the lens sufficiently so the wafer is moved easily, otherwise 
when starting it the wafer will crack and a part remain on the 
base lens. 
After the wafer has been transferred to the base lens it shoud 
be placed in position and cleaned slightly. It will then be neces- 
sary to neutralize the reading portion, as it may be found wavy, 
or not the correct focus. It must be remembered these wafers 
are extremely thin, consequently are easily bent. 
As it is necessary to hold the wafer with a stick when sliding 
off the backing it is not unlikely that the center was pressed down, 
thereby making the power .12 to .25 D. weaker, or possibly 
wavy. By heating the lens slightly and, if necessary, moving it 
around gently, there will be no difficulty in producing a perfect 
lens. Do not, under any circumstances, attempt to make these 
lenses without neutralizing the reading portion after heating each 138 
MECHANICAL OPTICS 
time. This also applies when recementing these lenses, and if 
the prescription does not accompany the job neutralize them 
before removing the wafers and make a note of the focus. When 
the wafers are in the correct position and neutralized correctly 
the bottom can be cut off and edged the same as any cement 
bifocal. 
In cleaning be very careful not to chip the edges and when 
wiping always wipe away from the edge of the wafer and not 
towards it. If the wafer should become chipped it can be trans- 
ferred to the backing and ground out. Do not attempt to grind 
or clean the wafer when not on the backing, as this is practically 
impossible. 
Some opticians are making a specialty of wafers ground slightly 
thinner than the regular cement bifocals. Wafers for this purpose 
are ground small in surfacing-usually about 22 mm. diameter. 
They are then edged to 18 mm., which is the usual size. While 
this can be done it does not produce a very satisfactory lens and 
at the same time requires more time. 
For Neufocal and thin wafer work "cooked" cement will be 
found more convenient and it also simplifies the work consider- 
ably. When the cement has been prepared so that it requires no 
attention it is possible to do better work on the lenses. With this 
method each job is cemented alike, and if the cement has been 
properly prepared you are reasonably sure of its giving satisfac- 
tion. Occasionally we hear of substitutes for balsam, but as yet 
there is nothing on the market that is particularly satisfactory. 
All of these preparations adhere to the surface of the lenses, so 
they either become more or less opaque or else the wafers cannot 
be removed. 
A wafer having the same appearance can be ground from a 
regular wafer, and this is called the horseshoe shape. It is no 
more invisible, however, on account of the thickness. If special 
wafers are ground for this purpose as thin as possible, and from 
22 to 25 mm. diameter, it is possible to make a very fair job. 
The prescription houses usually charge extra over the regular 
cement price for these lenses. The best way to handle these is 
to attach them to the end of a stick, about the size of a pencil, 
with sealing wax. They should be edged carefully to 18 mm. 
diameter. They are then cemented to the distance lenses in the 
regular way, but so that the top will be 2 mm. below the center MECHANICAL OPTICS 
139 
of the distance (Figure 150). After the lenses are cool, the bottom 
is cut off and ground, as usual. There is an occasional call for 
round wafers, usually about 10 mm. diameter. These can be 
Figure 150 
Figure 151 
ground in the same way, but cemented to the distance about 2 
mm. from the lower edge (Figure 151). Oval wafers are also 
ground in the same way. 
Perfection Bifocals 
Perfection bifocals are made by cutting out the lower portion 
and inserting the reading lens. The distance lens is first ground 
to size and the lower part cut with a hand diamond and a round 
pattern. It is then carefully broken out and ground on a small 
alundum stone. These stones can be obtained to fit any buff 
head and in any size. The regulation shape is one inch diameter, 
although occasionally, when a larger lower is desired, 1% inch 
is used. The lower lens is then cut the same shape and ground 
on the regular stone to fit this curve. Although these can be 
ground into the frames separately, it is better to stick them 
together with white sealing wax. If this method is used, the wax 
must be left in (and this is no objection). If cleaned out the lens 
will be too small for the frame. 
Grooved Perfection bifocals have been used to some extent 
and the advantage claimed is that the parts are interchangeable 
and can be made up from stock very quickly. The line between 
the distance and reading is quite conspicuous, however, and for 
this reason they have never had a large sale, especially since the 
invention of invisible bifocals. 140 
MECHANICAL OPTICS 
The distance lenses can be ground the same as the regular Per- 
fection, except that the curve is beveled instead of flat. This 
can be done on the small stone without any difficulty. The lower 
part is then ground flat to fit the curve of the upper and a groove 
made with a rat-tail file and drilling fluid. 
Perfection bifocals are rarely used except in cases where a 
patient is obliged to work near heat and cannot afford the fused 
or one-piece style lenses. It is also impractical to make them in 
frameless. 
Split Bifocals 
Split bifocals can be centered or not as you prefer. Ordinarily 
they are not, and when ground in this way the four pieces can 
be cut from two lenses, provided the power is the same in both 
eyes. The lenses are first split in the center and the straight 
edges ground on the side of the stone. Composition stones are 
molded and the sides are usually smooth enough. If not, the 
right side should be honed the same as the face. If a Craigleith 
is used the side will have to be turned true with a diamond and 
honed with a piece of Craigleith. When the straight edges are 
ground and beveled slightly to take off the sharp edge, they 
should be stuck together with white sealing wax. They can then 
be handled as one lens and cut to size with a pattern. They are 
also ground the same as any lens. These lenses can also be 
ground separately if desired, but it is more difficult to hold them 
when trying in the frame. If centered lenses are required the 
halves should be cut from the center of the lens and a pattern 
having this shape will be required. 
Whole bifocals are surface ground and are treated the same 
as an ordinary lens when ground to size. These are not recom- 
mended on account of the prismatic effect. 
Fused and One-Piece Bifocals 
Fused and one-piece bifocals are a surface-grinding proposition 
and will be described under that heading later. If care is used 
it is not difficult to surface them. At present, however, we will 
suppose that you prefer to buy them uncut and edge them your- 
self. When ordering them from the supply house, full informa- 
tion must be given, such as distance and reading powers, diameter 
and height of reading portion, axis, frame or frameless, if reading MECHANICAL OPTICS 
141 
part is to be set in and size of eye. When the lenses are received 
the axis should be marked and a cutting line drawn. The reading 
portion is then dotted with ink around the edge so that the circle 
can be seen easily. The usual diameter is 20 mm., and the height 
is within 2 mm. of the center. After the axis and position of the 
disk are inspected the lens is cut by pattern. Although these 
lenses can be edged by machine, it is recommended that they be 
ground by hand. The position of the disks can then be watched 
and there will be less liability of spoiling them. Also be particular 
that the disk is not scratched as flint is much softer than crown 
glass. In edging these lenses the disks should be first considered; 
that is, the shaping of the lens should be done on the lower edge 
first to make the disks the same height. After these have been 
made just right, proceed to shape up the other part of the lenses. 
In making the various combinations it is necessary to use flint 
of different indices and some of these grind quite hard. For 
these lenses craigleith stones will be found more satisfactory. CHAPTER X 
Frame Bending and Truing 
BRIDGES can be bent before or after mounting, and, al- 
though there is really no preference, some opticians obtain 
better results one way and some another. When bending 
an unmounted frameless bridge, or a frame before the lenses are 
inserted, gripping pliers can be used to hold the strap or eye 
wire so that the operation is simplified. An experienced person, 
however, can bend just as well after mounting, but the bridge 
must be held in the fingers. Under no circumstances should the 
spectacles be held by the lenses as they will surely be broken. 
Bridge bending is not hard to learn, but simply requires prac- 
tice and a good eye. It is discouraging at the first attempt, and 
the reason is often that the beginner selects a difficult frame to 
bend, and possibly one that could not be bent successfully any- 
way. In starting this work, select some old frames of soft metal, 
such as German silver or Roman alloy, as these make the best 
frames to practice on, and always have some dimension to work 
to, as nothing can be accomplished by bending at random. 
For the first operation take a frame, say 60 mm. P. D., and 
bend it to 58. This is done by holding the bridge with a pair of 
No. 36 pliers in the left hand and gripping the shank of the bridge 
with a pair of round or snipe-nose pliers about half-way from the 
eye wire to the turn, and, with a slight twist to the right, the eye 
is bent toward you (Figure 152). Then grasp the shank near the 
eye wire and bend the eye back, and you find that the shank has 
the desired bend and the eye has been thrown in quite a little. 
The amount, of course, depends on the twist. 
After bending this to the required P. D., bend it back again to 
the original shape. This will give you practice in widening the 
P. D. This is accomplished by straightening the shank with a 
pair of snipe-nose pliers; in other words, take out the curve known 
as the 47 style. In this operation the eye is thrown forward and 
it should then be brought into line with the No. 36 pliers. 
A frameless bridge is bent the same way, except that it is neces- 
142 MECHANICAL OPTICS 
143 
sary to have different pliers to hold the strap, and for this purpose 
the No. 39 angling pliers are the best (Figure 153). 
Next try raising and lowering the bridge. This is done by 
holding the eye-wire with the No. 36 pliers or the snipe-nose in 
Figure 152 
Figure 153 
the right hand and, grasping the crest of the bridge with the 
thumb and forefinger of the left hand, bending it up or down. 
This, of course, changes the angle of the crest a little, also the 
inset or outset, still it is the first step necessary (see Figure 154). 
If you desire to lower the bridge and keep the angle the same, 
bend the bridge down, as described in Figure 153, then grasp 
Figure 154 
Figiire 155 
the frame with the No. 36 pliers (if frameless, the No. 39 pliers) 
in the right, and the shank of the bridge with a flat-nose pliers 
in the left hand, bend the shanks downward (see Figure 155). It 
is impossible to raise the bridge and keep the angle of the crest 
and the inset or outset the same, except by shortening the shanks, 
and this should not be attempted until the first operations are 
mastered. 144 
JIEC7L42VZC4L OPTICS 
To widen the base take a pair of flat or snipe-nose pliers, and 
place a piece of cloth between the jaws to prevent marring the 
bridge, and flatten it to the required measurements and shape; 
then straighten the frame and you will find that the bridge is 
right, but the P. D. is wide. The shank can then be bent, as 
explained before, to shorten the P. D. 
To widen or narrow a pupilary distance a No. 53 plier has 
proven very satisfactory. This plier has one concave and one 
round-nose jaw and will give a perfect 47 bend to narrow or draw 
in P. D. 
Figtire 156 
A No. 57 plier (a new end-piece plier) has taken the place of a 
snipe-nose to some extent for angling or truing end pieces as it 
eliminates marring and when used in conjunction with a No. 40 
plier will produce a neat and finished pice of work (see Figure 
156). 
To increase the outset it is necessary to shorten the shanks, 
and it is quite difficult to do this without marring the stock, but 
if you go slow, bending a little at a time, this can be done all 
right. The first operation is to grasp the bridge with the thumb 
and forefinger of the left hand and, with a pair of round pliers 
in the right hand, gradually open up the turn, beginning with the 
small end of the pliers and gradually sliding the bridge toward 
the larger part of the pliers (see Figure 157). After this has been 
spread sufficiently, grasp the shank a little way from the turn 
and bend the eye straight, at the same time pulling forward on 
the bridge with the left hand. This pulls a little of the stock in 
the shank into the arch. It can then be trued up and the opera- 
tion repeated on the left side. 
To decrease the outset or increase the inset the shank must be 
lengthened and the first operation is to open the turn of the shank 
a little, as before, but, instead of grasping the shank back from MECHANICAL OPTICS 
145 
the turn, grasp it near the turn, or, if possible, in the turn, bend 
the eye straight and true up. Repeat the operation on the left 
side. 
Figure 157 
After practicing with the sample frame select a good frame to 
bend to measurements. The principal part of selecting frames 
Figtire 158 
for prescription jobs is in using good judgment, and you should 
train yourself to judge whether a frame can be bent to the dimen- 
sions required or not. If you do not learn this you can never 
learn to bend and will spoil stock, also waste time. In selecting 
a frame try and get everything right but one dimension, if pos- 
sible, and it is best to change the P. D., as this is easier than any- 
thing else. Do not take up a frame at random and expect to 
bend it to fit the required dimensions, but study it carefully and 
learn to judge it before touching it with the pliers. 146 
MECHANICAL OPTICS 
After the lenses are mounted they must be trued up, and, if 
the bridge has been bent previously, it is a very simple operation. 
If it has not been bent it is best to straighten it somewhat and 
lay it on the pupillometer to get an idea of what is to be done. 
The first operation is to get the lenses in the same plane, so that 
they will lie flat. Then turn the spectacles over, holding them 
endwise and sight across the outside surfaces. The four glass 
screws should then be brought into line (Figure 158). The 
spectacles should then be measured for all dimensions and any 
necessary alterations made at this point. 
We now suppose that the front is correct and true, and the 
attention turned to the temples. First straighten and true them 
so that the curve will be uniform. Then hold the front endwise 
with the temples upward, and in almost every case they will be 
found with one angled one way and the other in the opposite 
direction (Figure 159). A pair of parallel jaw pliers (No. 57) 
should then be used to grip the end-piece close to the strap or 
eye-wire, and, with a pair of No. 40 pliers, the joint is angled 
to make the temple perpendicular (Figure 160). The temple is 
then shut down, and, if it is not a line with the end-pieces, it is 
turned to the right or left, both pliers being used in the same 
position. This operation is then repeated on the other joint. 
If the front is to be angular, the joints are tilted in the same 
manner. 
Should it be necessary to set the temples back, the tip can be 
filed at the joint if gold or steel, but in gold-filled it is better to 
hold the joint with a pair of pliers and pull the temple outward 
with the fingers. Although one may be broken occasionally, it 
saves filing the gold, which is very important when handling 
filled goods. There are no pliers on the market for holding the 
end-piece in this operation, but they can be made specially for 
any make or style of joint. 
In truing eyeglasses there is very little to be done except to 
straighten the lenses so that they are in the same plane, and 
adjust them so that the droop will be right. This is a very impor- 
tant feature and many opticians do not understand that an eye- 
glass should be placed as far back on the nose as possible. A 
great many times a patient requests that the spring be tightened, 
when in reality it should be opened more to allow the guards to 
go back farther. Each time the spring is tightened the glasses MECHANICAL OPTICS 
147 
are thrown farther forward and, finally, they will grip only on 
the very edge. Now, in truing, the lenses are allowed to droop 
a little so that when placed on the nose they will be horizontal. 
Figure 159 
If they were in this position before placed on the face, or if the 
guards are not spread enough, the lenses will set upward. This 
not only looks badly but the lenses will be too high. A great 
many times eyeglasses are sent to the prescription houses for 
drop studs or guards to set the lenses lower, or even for new 
lenses drilled above center, when all that is necessary to make the 
glasses right is to droop the lenses a little more. 
When lenses are drilled above center they must always be 
turned down. For this operation grasp the stud with a pair of 
stud pliers in the right hand and hold the spring with the left. 
By bending the spring away from the lenses it will give them 
the required droop. Then take hold of the studs with both hands 
and open the spring a little until the lenses appear to be in about 
the right position. The guards should then be adjusted so they 
are parallel. 
For truing a finger-piece mounting use the No. 40 pliers, placing 
the jaw marked "top" with the large hole over the head of the 
screw on which the spring is coiled. The lenses can then be 
changed to any position without damaging the mounting (Figure 
160). 
When it becomes necessary to drop the lenses in an eyeglass 
frame to make them lower on the face, the screws should be 148 
MECHANICAL OPTICS 
loosened in the end-pieces and the lenses twisted so that they set 
upward considerably. Then grasp the studs with the stud pliers 
in the same manner as before described for frameless, and twist 
the eye downward, etc. If the lenses need to be raised, they 
should be twisted downward in the frames, the studs grasped 
with the stud pliers, and the spring bent backward toward the 
lenses, thereby raising the eye to the required position. This is 
another case that occurs frequently in prescription work where 
Figure 160 
frames are ordered with studs above center. If the lenses are 
fitted, they can be adjusted easily and no explanation is required; 
but if the frames only are ordered, it is almost impossible to shape 
them so that they look right, and, even if this is possible, the 
optician can easily change the position in fitting them up. 
Toric lenses can be adjusted so that they curve slightly to con- 
form to the features. If there is too much curve, however, it may 
be objectionable to the patient. It is well to remember this point, 
as frequently people complain that they are unable to wear 
Figure 161 
torics, and, if the glasses are adjusted so that the angle is changed, 
it makes them entirely satisfactory. MECHANICAL OPTICS 
149 
This also applies to the angle for reading, but not so much with 
torics as flat lenses. According to theory all glasses should be an- 
gled slightly, and the amount depends somewhat on the features. 
Lenses should never be drilled below center. If the spectacles 
are too low on the face, the bridge can be altered to raise them 
even if it is necessary to make one to order. It is seldom that a 
regular bridge cannot be used by dropping the shanks. 
If eyeglasses are too low they should be raised by changing the 
guards to some other style. In many cases a regular offset guard 
with a short blade will accomplish this result. It should be 
remembered that to raise the lenses the guard should be lower 
and to lower the lenses the guards should be higher. 
There are so many guards on the market that it is well to look 
the common styles over, and it will be noticed that the arms 
are set very differently on different styles. If one is familiar 
with these styles it is a very easy matter to select a different one 
to accomplish whatever result is required. When there is diffi- 
culty in fitting glasses so that they will be low enough, the drop- 
eye lenses sometimes are a great help, especially when the eyes 
are deep set. 
In truing temples do not leave too much curve to them, but 
have them so that they will be straight to the turn so that all the 
curve will be behind the ear. The right way to fit a temple is 
to make quite a sharp turn at ear (Figure 161). These not only 
prove more comfortable but they look better. When the nose is 
sensitive, cylinder bridges, cork or shell nose guards can be used, 
but when fitting the latter the bridge must be somewhat higher 
to allow for the thickness of the cork or shell. CHAPTER XI 
Surface Grinding 
SURFACE grinding is, in reality, a separate branch of me- 
chanical optics, for the reason that few opticians doing 
their own work attempt to do this work. Usually an 
optician puts in a plant to do his own edging, and for this part 
of the work he has time enough, but it is not practicable 
to surface the few odd combinations that he may be called upon 
to supply and attend to his trade at the same time. The right 
way is to install the outfit for edging, and, as his work increases, 
he can afford to hire a man for this part of the business. He can 
then install the surface plant for the accommodation of his 
customers or for the advertising feature. 
It requires very little time to grind an ordinary lens if the 
workman is efficient, and it often saves a delay of twenty-four 
to thirty-six hours if it is necessary to send out of town to a 
prescription house. 
The simple surfacing machines have one speed, and although 
this is sufficient, it is often convenient to have two speeds (Figure 
162). If one speed only is used, 900 to 1200 revolutions per 
minute are most satisfactory. If two speeds are used, 600 and 
1200 are the most common. 
A lens can be ground using almost any speed, but, of course, 
the higher the speed the faster the grinding, although it requires 
more attention as it is also necessary to feed the emery faster. 
There is a limit as to speed, however; if it runs too fast it throws 
the emery off quickly and you then are grinding on the tool, and 
in this way nothing is accomplished. It must be remembered 
that it is the emery that grinds and not the tool. 
The electrical-driven machine is very convenient as the motor 
is connected to machine by only one belt and, consequently, 
does not require countershafts. The one shown in Figure 163 
is a one-spindle machine for hand-grinding, but this make of 
surfacer is constructed on the independent unit plan so that 
any desired number may be mounted on one bench and indi- 
150 MECHANICAL OPTICS 
151 
vidually controlled. By removing the handle and substituting 
an automatic attachment the operator can immediately convert 
the machine from a hand-grinding into an automatic polishing 
Figure 162 
machine. It also not only requires very little power to operate 
it, but when the machine is stopped it is using no current. 
Automatic work is somewhat slower, and most opticians having 
these machines use this attachment for polishing only. 
The tools cost about two dollars per pair and the gages about 
sixty-five cents per pair. These are not supplied with the 
machine, so it is necessary to order whatever powers will be 152 
MECHANICAL OPTICS 
needed. It is well to have each .12 to 3 D., then .25 to 8 D., then 
.50 to 14 D., and each diopter to 20 D.; also a piano and an 
extra 6 D. for roughing torics. It is also necessary to have a 
pair of gages for each power. 
Another point that must be considered is the index of the 
glass used. Tools are regularly made for 1.523 index; if other 
index is used, it will be necessary to have a set of tools for it, 
although by making an allowance it is possible to work very 
closely on one set of tools. There is about .03 D. difference on 
each diopter, and, as this is so small, it wall come near enough 
up to 2 D., but above that select the nearest tool and polish 
it stronger or weaker as the case requires. (This will be explained 
later.) 
Figure 163 
For grinding material the following grades of emery will 
be required: No. 70 for roughing, No. 100 for smoothing, No. 
4-F for finishing and a grade of washed emery for fine finishing. 
The first three mentioned should be bought in ten-pound cans; 
the washed can only be obtained from optical concerns in five- 
pound cans. This is also very expensive, as compared with other 
grades, but it lasts a long time as it is necessary to use only a 
very small quantity at a time. It should also be remembered 
that a good finish saves a great deal of time in polishing. MECHANICAL OPTICS 
153 
There are several kinds of grinding material on the market, 
such as carborundum, corundum, alundum and crushed steel, and 
these are graded about the same as emery. They cost more, but 
are supposed to cut faster and stay sharp longer, but for a single 
machine it hardly pays to mix the different materials. Alu- 
minum oxide is a branded abrasive used very largely by the 
manufacturing opticians doing surface work. This material is 
Figure 164 
the product of an electric furnace and is made from Bauxite. 
It is melted at a temperature of approximately 3500 degrees 
centigrade, where it is drawn off in the form of "pigs," which 
are then crushed and graded into the various sizes used by the 
wheel manufacturers, polishing trade, and the optical trade. 
Generally the optical trade uses two sizes. On coarse work they 
use either No. 60, 70, or 80. For the next operation they use 
No. 180, 200, or 220. From this point they use a different 
product which is generally known as 10, 20, or 30 minute Turkish 
Emery. 
It should be remembered that the sharper or rougher the 
materials used the harder it is on the tools and they must be 
watched more carefully to see that they are kept true. For 154 
MECHANICAL OPTICS 
polishing, rouge is used, and this comes in several grades. The 
grade used by supply houses is all right and should be ordered in 
five-pound packages. There is also a black pigment used by 
some opticians. For polishing, cloth, felt or broadcloth is most 
commonly used, and, although this can be obtained almost every- 
where, it is more satisfactory to order it from the supply house 
and you will then be sure of getting a quality that will do satis- 
factory work. Pitch can be obtained in packages from the whole- 
saler, and this is used to stick the lenses on to the blocks. It can 
be made easily by taking a quantity of resin and melting it. 
When it is poured off to cool, a little turpentine should be added 
to soften it, or, in other words, to keep it from getting brittle. 
In the winter it should be softer than in the summer, as the 
temperature will affect it. If it is too soft the glass will slide on 
the block in polishing, and if too hard it will jump off the block 
when chilled in water. To make it black a little lampblack can 
be added. The dark surface makes it much easier to inspect the 
lenses when polishing. 
Figtire 165 
A few blocks are usually furnished with a machine, but more 
will be required. Those furnished are round and flat (Figure 
165), and these will do for weak power lenses. For stronger 
powers deeper holes are needed, so they are made with a lug on 
the back (Figure 166). Some of these should have a convex face 
Figure 166 
(Figure 167) and some concave for torics (Figure 168). If square 
blocks are used it will be much easier to caliper the lens while 
grinding. A few small, round blocks will also be needed for wafers 
(Figure 169). MECHANICAL OPTICS 
155 
Before fitting the tool to the machine it should be tried with 
the gage, and if it is not perfectly true it should be made so 
before attempting to grind the lens, otherwise the power will 
Figure 167 
not be correct. A great many opticians use tools without gages, 
or, even if they do have them, they allow the tools to get all out 
of true and then send them to a machinist to be put in shape. 
This is entirely wrong and unnecessary, as it is a very easy matter 
to true the tool a little each time. The method is the same as 
Figlire 168 
trying the surface of a board to see if it is square; that is, the 
tool is held in one hand on a level with the eye and the gage is 
held perpendicular to the surface. If no light can be seen it will 
fit perfectly; if light is seen, notice whether it is weaker or 
stronger or if there is simply a high or low spot. In grinding, 
Figure 169 
usually the tool wears on the side and the center will be high. 
For this work a piece of carborundum 8 inches long, 2 inches wide 
and 1 inch thick is used, of a fairly coarse grade. The tool is placed 
on the spindle and run at full speed. With the carborundum 
stick it can be turned nearly as well as in a lathe. A little should 
be taken off at a time, however, and tried frequently with the 
gage. 156 
MECHANICAL OPTICS 
There are usually two pans furnished with the machine; 
one of these should be used for grinding and the other for polish- 
ing. Some rough emery (No. 70) is placed in the pan with 
quite a lot of water, enough so that it covers the emery. It will 
do no harm if there is too much as the emery will stay at the 
bottom. In roughing, this is fed on to the tool with a spoon or 
with the fingers. The No. 180 emery should be placed in an agate 
drinking cup, or something of that nature, and wet so that it 
will be in the form of a paste. The No. 4-F emery can also 
be placed in a cup of this kind, but the washed should be kept 
in a small jar or box with a cover. The rouge can be placed 
in a cup and wet in the same way. An extra cup of water should 
be kept at side of this, and, in polishing, this is applied with a 
brush. It will be necessary to have a large pail of water handy 
for rinsing the lens from time to time, and, after these different 
receptacles are arranged conveniently, you will be ready to grind. 
For surface grinding it is necessary to have a stock of rough 
cylinders. These come in thickness varying from 2 to 10 mm. 
Those most commonly used are and 4 mm. In selecting 
the stock for any particular job it is well to caliper them, as this 
not only saves glass, but also saves a great deal of time in rough- 
ing. For this work a pair of calipers graduated in 1/10 mm. is 
Figure 170 
used (Figure 170). In construction this caliper differs from those 
heretofore offered, insuring long life and absolute accuracy 
throughout. The "U" form offers strength and stability and 
the large bearing surface is made of bronze to obviate excessive 
wear at this important point. 
It is necessary to allow 4/10 mm. for each diopter and 8/10 
mm. for each prism diopter. To this is added about 4/10 mm. 
for grinding. For example, if we are to grind + 1 O + .50, a + MECHANICAL OPTICS 
157 
.50 cylinder is selected 8/10 mm. thicker on the edge than the 
required thickness of the finished lens, thus allowing 4/10 mm. 
for the curve and 4/10 mm. for the grinding. When grinding a 
convex lens the center is hardly touched, provided the stock 
is about the right thickness at the start, but, unless the lens is 
fined down very carefully, which takes extra time, you cannot 
help grinding off a little extra glass, and, for this reason, a little 
allowance for grinding must be made. If the combination to 
be ground were - 1 O - .50, a rough cylinder having about the 
right thickness on the edge would be selected as practically the 
center only is ground. The allowance for grinding is all that 
would be taken off the edge. If the combination to be ground 
were + 1 O + .50 axis 90 O 1° in, it would be necessary to allow 
8/10 mm. more for the prisms. For example, the lens when 
finished should be 2 mm. thickness on the edge; to this is added 
4/10 mm. for the one diopter and 8/10 mm. for the prism. To 
this is added 4/10 for grinding and the rough cylinder to be se- 
lected would be 3 6/10 mm. For this it would probably be neces- 
sary to use 4 mm. stock. It should be remembered that over 
4 mm. rough cylinders cost extra for each millimeter thickness. 
Toric rough cylinders should be purchased in molded form, 
when possible, as this saves a great deal of roughing. Almost all 
blanks are made this way at the present time, but some of the 
smaller manufacturers put them out in flat form. There is also 
another point to be considered in using these lenses, and that is 
that often it is not possible to get the thickness out. In other 
words, when they are not molded, so much stock has to be ground 
out inside that it leaves the lens a knife edge. These can be 
obtained in plus and minus 6 D. base curve, also plus and minus 
9 D. base curve. The majority of combinations can be ground 
on + 6 D. base, but, as the spherical power increases, it reduces 
the toric effect so that it becomes necessary to transpose the com- 
bination, so that it is ground on a - 6 D. base. It is preferable, 
however, to grind these combinations on a 9 D. base. For ex- 
ample, + 30 + 1, if ground on a + 6 D. base, would have but 
- 3 inside curve. If ground ona + 9 D. base curve it would be 
- 6 D. on the inside. It could be transposed, however, and 
ground on a - 6 D. base curve and it would be - 6 D. on the 
inside and + 10 D. on the outside. It should be remembered 
that the base curve is always the weakest meridian on the cylinder 158 
MECHANICAL OPTICS 
side; that is, a + 6 D. base curve is + 6 in one meridian and 
stronger in the other. The difference will be the power of the 
cylinder. A - 6 D. base is - 6 D. in one meridian and stronger 
in the other. The spherical power is obtained by grinding the 
opposite side. 
Another point to keep in mind is that concave toric cylinders 
cost more than convex, and, whenever possible, all combinations 
should be transposed so that a plus cylinder can be used. For 
example,in the combination - IO - .50,if it is ground as written 
a - .50 toric cylinder would be selected and the cylinder surface 
would, of course, be on the inside. On the outside would then 
be ground + 5, thus making the - 1 sphere. If this were 
transposed to - 1.50 O + 50, a 4- .50 toric cylinder would be 
selected and - 7.50 ground on the inside to produce the - 1.50 
sphere. The reason that concave toric cylinders cost more is that 
the large factories have not as yet perfected machines for grind- 
ing these in quantity the same as the convex, and, consequently, 
they are ground singly at a greater expense. 
When selecting colored lenses it is necessary to have in mind 
that as the glass is ground thinner the shade will be much lighter, 
and it is a very difficult matter to judge just how they are coming 
out. For this reason colored lenses with power are always 
expensive as a great many times it is necessary to grind two 
or three pairs before the exact shade is produced. The de- 
mand for colored lenses, however, is so unimportant that they 
call for little consideration as compared with the colorless variety. 
For wafers, spherical lenses are used, but always selected 
so that one surface is correct. For this Pcx. lenses work 
in nicely as the old curves can always be found. For example, for 
the combination - 1 O + 3.50, a + 2.25 Pcx. would be selected, 
and this would be on the outside + 3.50 and on the inside 
- 125. We would then block it with the convex surface down 
and regrind the inner to - 1, at the same time reducing the 
thickness to whatever is desired. 
Spherical prisms can be ground on rough prisms or a stronger 
piano prism can be used to get the thickness required. 
Marking the Lens 
The machine now being in operation, and the emery arranged 
conveniently, the first operation will be to mark the lens. If a MECHANICAL OPTICS 
159 
brown pitch is used in blocking, black ink will show well enough, 
but if black pitch is used, white ink is much better. If a simple 
compound is to be ground, no marks are required, but with all 
prism combinations the axis must be marked. This method is 
the same as for cutting, except that it must be taken into con- 
sideration that when the lens is on the block, you, perhaps, are 
looking at it from the wrong side. This, of course, depends on 
the combination. For example, if the combination to be ground 
is - 50 axis 45 O 1° in, the lens can be marked with the cylinder 
in (or up, as it is laid on the protractor) just as it is to be fitted 
to the spectacle. 
It is, of course, necessary to mark all lenses on the cylinder 
side, otherwise the marks will be ground off. When this is stuck 
on the block the cylinder will be down, and you are really work- 
ing on it as though the axis was 135°. On the other hand, 
if the combination is + 50 axis 45 O 1° in, it would be necessary 
to mark it as if it were to be 135°. The reason for this is that 
you are marking it on the cylinder side, and after this is com- 
pleted, it is to be fitted to the spectacle with this side out. The 
reason for marking is to have a line to grind the prism on, in 
other words, to tell the direction of the base. If this is to be in, 
the axis must be set at the proper angle so that the prism line 
must be horizontal. It does not matter, however, how this is 
placed on the block as long as the relative position of the axis to 
the base is correct. 
If a double prism is to be ground, that is, a lens with the 
prism out and up, etc., it will be necessary to have a line for both 
prisms. In marking, the axis is first dotted and laid on the pro- 
tractor at the proper angle, the prism lines are then drawn and 
the bases marked with the letter "B". As the prism lines are 
the only ones that are considered, the axis dots can then be 
rubbed out if preferred to save confusion. There is a chart 
published for calculating the effect of double prisms that is very 
convenient, but it is not absolutely necessary to use this in surface 
grinding. A double prism is really a single prism ground at a 
different angle. For example, a 2A up = 2 A out in the right eye 
wTould be practically a 3 A axis 135°. 160 
MECHANICAL OPTICS 
Blocking 
For blocking, an ordinary cheap gas stove is sufficient, but 
a piece of sheet iron should be laid over the top to prevent any 
pitch running down into the stove and clogging the holes. The 
blocks should be heated on the stove, and, when quite hot, 
should be taken off and placed on the bench. A little pitch should 
then be put on and allowed to melt. The glass to be blocked 
should be warmed slightly, laid on the block a second and then 
removed. It will be noticed that some of the pitch sticks to 
the surface. A little more of the pitch should then be put on 
the block and the lens laid on a second time, and then removed. 
In this way you gradually build up a backing for the lens. This 
operation should be repeated several times until there is pitch 
enough between the block and the lens so that it will hold se- 
curely, and also that there is no danger of the glass touching 
the iron. This not only prevents scratching, but if there is not 
pitch enough to form a good bed for the glass, it will crack 
when pressure is applied in grinding. 
Flat lenses do not require as much pitch as those having a 
concave surface or a toric. Ordinary concave lenses can be 
blocked on flat blocks, but care must be used to fill up the space 
with pitch for the reason just stated. For convex toric cylinders, 
a block having a concave surface should be used, and although 
the curve should be about the same, it is not absolutely necessary, 
as the surface can be built up with pitch. For concave toric 
cylinders, convex blocks should, of course, be used. After the 
lenses are blocked, they should be allowed to cool gradually. 
After they are cool, the pitch on the back of the lens that over- 
hangs the block should be scraped off with a knife so that the 
the four sides can be measured with the calipers. Lenses can 
also be blocked over an ordinary Bunsen burner, and if it is 
done in this way, the pitch should be moulded into sticks and 
melted on the block the same as sealing wax is used. 
We will suppose that the combination to be ground is + 1 
O + 50 cyl. and IX strap on the edge when finished. We 
have selected a + 50 cylinder, 2X mm. thickness, and have 
blocked it as described. We then select the tool to grind + 
Surfacing MECHANICAL OPTICS 
161 
1 sph. This, of course, will be a concave tool to grind a convex 
surface. It should be tried with the gage to see if it is correct. 
The rough emery is already in the pan, with plenty of water. 
The block or lens is then taken in the left hand and held on the 
tool, spooning the emery on with the right hand. After the sharp 
edges are ground off, the block can then be placed under the 
spindle in the handle. It will then be necessary to adjust this so 
the lens is in the center of the tool. As to position, the grinder 
can stand wherever it is most natural. Some of the older ma- 
chines are arranged so that the handle points directly to the 
operator, and some use it in this position; others stand a little to 
one side. In either case this is somewhat awkward and the best 
way is to have the handle run from right to left, so that it is in 
a horizontal position to the grinder. 
The newest machines are made so that they can be adjusted 
to any position. When the handle has been adjusted correctly, 
the machine can be started. The lens will then revolve or spin 
on the spindle, and the grinder then moves the handle forward 
and back so that the lens will travel from the center to the edge 
of the tool. This motion is to break up the rings which would 
form on the surface of the glass if it were held in one position. 
One must be careful, however, not to run over the center of the 
tool, or the block may fly off. As the lens is moved back and 
forth, the emery should be fed continually with the spoon. After 
the lens has ground a minute or two, it should be removed, and 
if it is ground all over the surface, it is ready to measure. Now 
take the calipers and measure all four edges. If it is the same 
all around, the lens is centered and not prismatic. If there is a 
difference, take the block in the hand the same as when starting 
and grind off a little of the thick edge, then measure again. If 
found correct, it can then be placed on the spindle again and 
ground as before. 
The system of measuring should be remembered, that is, 
.4 mm. allowance for every diopter. As we started with a lens 
lyi mm. thick and wish to obtain a lens 1%" mm. it will be neces- 
sary to grind off 1 mm. or .1 on the calipers. In the roughing 
about .4 mm. will be taken off, being careful, of course, to keep 
it centered as described. Do not be afraid to measure it too 
much, for it is better to take off a little at a time than to spoil the 
lens. After it has been roughed down, wipe off the tool and rinse 162 
MECHANICAL OPTICS 
the lens in water. (For this purpose, keep a pail at the side of 
the machine.) Now take the No. 180 emery, which is in the cup, 
and grind as before, smoothing the surface and taking off about 
.2 mm. Then wipe off the tool again, rinse the lens and take 
the No. 4-F emery. This you can apply with the fingers, or a 
brush if preferred. After this grade, clean the lens and tool 
again thoroughly and you are ready for the fine finishing. For 
this operation it is necessary to use but very little of the finest 
emery, applying it with one finger. The idea now is to get just 
as fine a surface as possible to obtain next to a polish. 
Although it is unnecessary to waste the emery, all scratches 
and pits must be removed, as time is not only wasted in trying 
to polish them out, but the len produced is not so good. It 
should also be remembered that this grade of emery is expensive. 
All through these different operations keep in mind that the lens 
must be calipered frequently to keep it from being prismatic, 
and also, that as you are continually grinding off glass, you are 
nearing the thickness required for the finished lens. Also be 
particular to clean the tool and the lens between the different 
grades of emery used, as one grain of rough emery on the tool, 
when fine finishing, will scratch the surface. 
If the combination was - 1 O - 50,the operation would have 
been the same, using, of course, the opposite tool, but if the 
order called for a certain thickness in the center, the lens would 
be .4 mm. thicker on the edge. Although this has to be taken 
into consideration in convex combinations, allowing the lens 
to be .4 mm. thinner on the edge than on the center, it does not 
make quite so much difference, for the reason that the concave 
lens is apt to be ground through the center. This will be noted 
in grinding the stronger combinations, and one must be accurate 
in measuring when selecting rough cylinders. 
For each prism diopter .8 mm. must be allowed, so if the 
combination to be ground was + 1 O + 50 axis 90° O 1° in, 
we would allow .4 mm. for the sph. and .8 mm. for the prism, 
making 1.2 mm. besides the allowance for grinding. 
Grinding Prisms 
We have already described the method of marking for a prism, 
and the blocking is the same as for a compound. In starting MECHANICAL OPTICS 
163 
to grind, the lens is held in the hand, not on the spindle as 
before, but we grind off the edge where the apex is to be, leaving 
the edge for the base as thick as possible. It will now be seen 
why it was necessary to mark a mechanical axis or prism line. 
It should be measured on this line, and the base should be 
4/5 mm. thicker than the apex. The two opposite sides are of 
no account, except that they should be exactly the same thickness. 
If one was thicker than the other, there would then be a prism 
power up or down as well. By this it will be seen how to grind 
a double prism. For example, if the combination was to be 
+ 10 + 50, axis 90° O 1A in and 1A up, we would have 4/5 mm. 
difference on the edge between the apex and base on the 180° 
line, and also 4/5 mm. difference on the 90° line. When the 
lens has been roughed down in the hand so that the prism power 
is approximately correct, it can be placed on the spindle and 
roughed down to the proper thickness. It should be remembered 
that all the prism power must be roughed on with the block 
in the hand, and that the lens cannot be ground prismatic on 
the spindle. This does not mean that the lens will not grind 
prismatic because it will grind off center more or less, but it 
means that you cannot control the direction of the prism power 
on the spindle. As different grades of emery are used, the 
edge must be measured constantly, and if there is any variation 
found, take the lens in the hand again and true it up with what- 
ever grade of emery you may be using. 
In grinding a cylinder prism, the plane tool is used, and 
the prism power only has to be considered. For example, a + 50 
axis 45 O in would be marked and blocked as usual, and the 
apex ground to the proper thickness, say 2 mm. The base should 
then be 2 4/5 mm. thickness. The plane tool should be kept as 
accurate as possible for the reason that the slightest spherical 
power can readily be detected on the axis of a piano cylinder. 
This is also a difficult lens to polish, as will be explained. It 
should be remembered that a piano lens is the most difficult to 
grind. 
In grinding the wafers, it is only necessary to grind them 
small enough to be thin. The spherical lens is blocked, putting 
the surface that is correct down on the block, of course, and 
grind it down to about 30 mm. diameter. For horseshoe-shaped 164 
MECHANICAL OPTICS 
wafers they can be ground smaller, but if less than 25 mm. they 
will be too thin to handle. 
Torics are ground the same as a regular compound, obtain- 
ing the rough toric cylinders from the supply house. If the com- 
bination to be ground is + 1 O 50, a + 50 toric rough cylinder 
is selected, and this will have the cylinder on the outside. This 
is blocked with the surface down, and - 5 ground on the inside. 
It should be remembered that these should be blocked on concave 
blocks so that the surface will fit. 
If any amount of roughing is to be done, do it on the extra 
tool kept for this purpose. This should be about 6 D. curve, and 
it is not necessary to keep it accurate. The idea is simply to have 
a tool with strong power to save the regular tools. Rough emery 
grinds the tool very quickly, and, consequently, throws it out 
of true. Quite a little time must then be spent to keep a tool 
in shape. 
Toric and Special Lenses 
Toric lenses are ground in the same way as compounds; that 
is, the spherical side. The rough toric cylinders are obtained 
from the wholesaler in the moulded form, and this reduces the 
labor considerably. When it was necessary to grind them from 
the flat form a great deal of roughing had to be done. The 
blocking has already been described, and, if this is properly done, 
there is very little danger of breaking. In grinding torics it is 
very difficult to get the spherical power accurate for the reason 
that in grinding strong curves a little variation can hardly be 
detected, but when the curve is ground on the opposite side 
the total power of the lens is so weak as compared to the surfaces 
ground, that it is a very easy matter to grind the power .12 D. 
weaker or stronger. For this reason torics should be ordered 
of reliable houses that give this matter proper attention. 
If a + 1 O + 50 cylinder is to be ground, a 4- 50 toric cylinder 
with + 6 D. base curve is selected, and - 5 D. spherical ground 
on the inside. For + 2 O + 50, a - 4 D. spherical would be 
ground on the inside, and so on. As the spherical power in- 
creases, the inside curve decreases, so that if the full toric effect 
is desired it will be necessary to transpose the combination and 
grind it on a concave cylinder. 
If a -1 O - 50 cylinder is to be ground, a - 50 cylinder with MECHANICAL OPTICS 
165 
- 6 D. curve is selected, and + 5 sph. ground on the outside. 
Remember that the base is always 6 D., and, to produce the re- 
quired spherical, simply deduct the power desired, and this will be 
the curve to grind. Also do not forget when grinding a + 
and + combination on a - cylinder that the combination must 
be transposed. This is the cause of many mistakes. For ex- 
ample: + 1 O + 50 cylinder if ground on a + cylinder will have 
a - 5 inside curve, or 1 D. weaker than the base. When trans- 
posed it will be + 1.50 O - 50, and the outside curve will be 
+ 7.50, or 1.50 D. stronger than the base. When 9 D. base 
Figure 171 
curve is required plus cylinders can almost always be used, and it 
is rarely necessary to use minus. The curves are not only ex- 
treme, but the rough concave cylinders are expensive. 
Toric cylinders are usually ground on automatic machines, 
although a hand attachment for the regular surface machines 
can be obtained (Figure 171). This work at present is not 
attempted by opticians, except in large shops or in localities where 
it requires some time to obtain the rough stock. 
The hand attachment requires special blocks, having an iron 
rod running through the center (Figure 172), to keep the axis 
of the lens in line with the tool. If the cylinder is fairly strong 
it will save time and also the wear on the tools to rough a spherical 
power on the glass first. It is then placed on the cylinder 
tool and the cylinder power put on with 3-F carborundum. This 166 
MECHANICAL OPTICS 
material cuts faster than emery and leaves a smooth surface 
The machine can be run at almost any speed, but for the be- 
ginner it is better to use about 600 revolutions per minute 
Figure 172 
It is operated by the hand lever, the same as when grinding 
a spherical, but it is necessary to stop the machine to feed it. 
Care must also be used to keep the lens on the tool, as a slip 
is apt to not only spoil the lens, but the arms of the machine 
can also be broken very easily. After the carborundum, No. 4-F 
emery can be used and then the washed emery. The operations 
are the same as when grinding sphericals, except that it requires 
a great deal more time. For those that can afford it, the auto- 
matic machines are to be preferred. 
A new machine that has been on the market for some time is 
the Wellsworth "M 27" (Figure 173). It will grind one surface 
at each operation accurately and with reasonable speed. 
Another machine that is used for cylinder and toric work 
quite extensively is the "Torcyl" (Figure 174.) This machine 
has a different action from the "M 27" in that the lap is station- 
ary, and the lens is moved sidewise by a horizontal arm, with 
provision to operate two lenses at the same time. 
Prescription houses are frequently asked if certain com- 
binations, especially prisms, can be made in torics. All combina- 
tions of lenses can be ground in toric form, but sometimes no 
better results are obtained. For example, - 12 sphere O - 1 
cylinder would be an extreme curve if ground in toric form, as 
the inside curve would be - 19 D. It would be much better to 
make it in double concave form with - 6 spherical on one side MECHANICAL OPTICS 
167 
and - 6 0 - 7 on the other. If this form was not desired, it 
would be better to make it in the regular compound form. 
Figure 173 
It is also a common idea that strong concave combinations 
are thinner when ground in toric form. This is wrong; these 
can be made lenticular, or ground on glass of a high index. A 
lenticular lens is one having the power in the center only, usually 
in a circle, having a diameter of 22 mm. This is the stock form, 
but they can be ground with the circle as large as the width of 
the lens. The larger the circle, the thicker the lens, however. 
Outside the circle is usually piano, but when a large circle is 
desired, the outside can be ground convex. This reduces the 
thickness on the edge, so that it may be as thin as desired. 
This style of lens can also be made with an oval center, but 
these are quite difficult, and it is best to order them from the 
prescription houses. Lenticular lenses are very desirable and 
should be used more. The reason that they are not is that 168 
MECHANICAL OPTICS 
opticians are not generally familiar with the different forms, or 
else they believe that the field is limited. 
In grinding lenses of this style the first operation is to grind 
the power in the center, the same as any regular lens. This 
may be ground away out to the edge if desired. It is perhaps 
just as well to do this when the power is not strong, but in the 
higher power lenses it requires too thick stock and it also makes 
unnecessary work. After the focus is ground and polished the 
center should be filled with sealing wax and then with the piano 
Figure 174 
tool grind off until the circle is the correct size. This should not 
be over 25 mm., unless the focus is not over 10 D. If higher 
than this, it makes a better lens to grind it convex. The power 
of the outside curve will depend on the size of the circle desired, 
also the focus of the lens. The curve required will vary from 
6 D. upward, and, until you are experienced, it is a good plan 
to select a low power and try it. If the curve is not steep enough, 
try a stronger one. The outside surface should be polished 
before the wax is removed. In referring to the outside curve 
of this style lens we mean the curve outside the circle and not 
on the opposite side of the lens. MECHANICAL OPTICS 
169 
Fused and One-Piece Bifocals 
Fused and one-piece style bifocals are very interesting to grind, 
and as the blanks can be obtained from your supply house, the 
most difficult part has already been done. In ordering the blanks, 
it is necessary to give the full prescription ordered by number 
from a chart, so that the proper blank can be selected. It must 
be remembered that the curve ground on the disk side will 
change the addition, so this must be taken into account. It is 
also impossible to grind a cylinder on the disk side for this 
reason, so in compounds an allowance for the spherical must be 
made. In making the blanks of fused bifocals, a depression is 
ground in the base lens of a certain power, the curve having 
been calculated for the index of the flint to be used for the disk. 
For example: If a 10 D. will produce 2.50 D. addition, each 
diopter will add + .25. If the blank was intended for a piano 
surface and a + 1 was ground on it, it would increase the read- 
ing addition .25 D. By this it will be seen that if the prescrip- 
tion called for distance + 1 sphere O - 50 cylinder axis 90, 
reading + 3.50, and the blank used was intended for a piano 
surface, the lens when finished would have for reading 4- 3.75. 
The blanks are blocked the same as any lens, but care must be 
used in heating. 
If the lens was not properly annealed it is very liable to break. 
The disk side is always ground first, as this must be brought 
down to the proper size. This is the most particular part of 
the work, as it can be easily ground too small. After the flint 
is ground off so that the surface is even it should be fined down 
very slowly, as this glass is very soft and grinds away quickly. 
In grinding a pair, always grind both disk surfaces first, so that 
the disks can be made the same size. Frequently, one lens will 
be ground so that the disk is the correct size, and then the other 
will have a scratch or imperfection in it, so that it must be 
ground smaller. It will then be necessary to reduce the other 
to match. In this way, grinding first one and then the other, 
it is possible to get them to match. After the disk side is ground, 
the other side can be brought down so as to make the finished 
lenses the proper thickness. If a cylinder is to be ground on 
the other side, it will be necessary to mark the axis, as the disk 
must be in the proper position. If the lens is twisted to correct 170 
MECHANICAL OPTICS 
the axis after the lens is finished, the disk will be out of center. 
If the axis is found to be off, it must be blocked again and 
reground. 
When the lens is laid out for axis, always try the tool with 
lens measure and see if the axis is in the proper position. There 
is nothing difficult in grinding these lenses, but grind slowly, 
taking off a little at a time. In polishing, remember that the 
flint is soft and is easily made wavy. 
When the lens is ground and the finish is as near perfect 
as possible, the lens is ready to polish. The tool should be 
cleansed and the lens rinsed in water. Then take a piece of felt, 
either round or square, as you prefer, and stick it on the tool. 
The material used is called "Tacky," and this is made from 
powdered rosin, or pitch, the same as used for blocking, and 
cut with alcohol, so that it is in the form of a paste. This is first 
applied to the surface of the tool with a brush, while it is running. 
The machine is then stopped and the felt pressed down firmly. 
Then start the machine and with a knife trim off the corners 
near the edge of the tool. This is where the art of polishing 
a lens, weak or strong, comes in. A convex lens may be polished 
strong by cutting the cloth large, and weak, by cutting the cloth 
small. A concave will be just the opposite: the lens will be 
strong when polished on a small cloth and weak on a large cloth. 
The rouge should be placed in a cup and mixed with water, 
so that it will be in the form of a paste; then place aside of this 
another cup containing plain water. Now apply the rouge with 
a brush, so that the felt will be well saturated; then place the 
lens on the spindle and start the machine. As the lens revolves, 
move the lever forward and back, the same as when grinding, to 
break up the rings that would form if it were held in one position. 
Apply the rouge occasionally, but do not let it get too dry, so 
that it will cake on the cloth. If it has this appearance, apply a 
little water. A lens will polish faster with a fairly dry cloth, but 
it cannot be too dry, of course. After polishing for two or 
three minutes, take the lens off the spindle and examine the sur- 
face. For this purpose a gas flame will give better results, as it 
has more or less flare. Daylight will answer, but electric light 
is not very good. The lens should be held in the hand, a little 
below the level of the eyes, in such a position that a good re- 
flection of the flame will be found on the surface of the glass. By MECHANICAL OPTICS 
171 
tilting it gradually so that the reflection moves across the surface, 
it will show up any imperfections, such as pits or scratches. 
When the glass has but a slight polish, the emery marks will be 
seen and these must be polished out. 
After the first inspection, and the lens has been found free 
from large pits and scratches, replace it on the spindle and polish 
a few minutes longer and then inspect again. As soon as the 
emery marks have all disappeared the lens is polished. If for 
any reason a scratch appears, or a pit of any size is found, it is 
best to finish it again with the washed emery, as this will not 
only save time, but a better lens will be produced. When a lens 
is polished too long, the power is apt to be off, so that the quicker 
it is polished the better. Remember, however, that it will take 
just about so much time to polish a lens, and that just putting 
a glaze on the surface is not a polish. Beginners are very liable 
to make this mistake, and it requires quite a little practice to 
judge the surface of a lens. The cloth can be used several times, 
and, in fact, until it wears down too thin. When polishing a piano 
surface, such as a cylinder prism, never use a new cloth, but al- 
ways use one that has been used before. The nap on a new one 
will sometimes produce aberration. 
To remove a lens from the block, place it under a running 
faucet for several minutes. If the pitch is then started a little 
with a knife, the glass will come off easily. The lens should 
then be placed in a basin of turpentine to eat off the pitch. 
Wellsworth Forty-Fives 
New developments in the manufacture of one-piece bifocal 
lenses have made it possible to produce a lens to take the place 
of the old style cement bifocal at very near the same price. 
They also eliminate the cloudy or discolored reading portion 
which is caused by poor or burned cement, chipped segments 
or the effect of atmospheric conditions on the cement, causing 
the wafer to loosen and let air under the surface. 
Clear Sight, or Wellsworth Forty-five, is one of these less ex- 
pensive high-grade bifocals obtained in several sizes and shapes 
of wafer, both round and regular shapes, so any desired width 
dr height can be obtained. These blanks are delivered by the 
manufacturer with the inner or wafer side finished, and are 172 
MECHANICAL OPTICS 
constructed on the - 4, - 6 and - 8 spherical curves from which 
all spherical or cylindrical combinations can be computed. They 
are furnished in all conditions from .50 to 4.50 with .25 diopter 
increases. 
In surface grinding the cylinder is always transposed to a + 
cylinder and ground on the outer or convex surface of the lens, 
the inner surface being finished as above stated. The I) should 
be carefully considered and the proper blank selected having the 
proper reading addition. The curves should be gaged with 
the lens measure to prove they are correct and to be certain the 
right blank has been selected, after which the blank is marked 
for surfacing, although in most cases it is more satisfactory to 
order the finished lens uncut from your prescription house. 
Good results can be obtained by using a little care and judgment. 
The dividing line between the distance and reading portion 
should be outlined by a drawn line, using white ink applied by 
using a steel pen or pointed piece of wood, the white ink being 
better as it shows up more distinctly with the black pitch when 
cemented on the block preparatory to grinding. The blank 
should then be placed on the protractor card, convex side down. 
The height of wafer, or reading portion, should then be de- 
termined and a dot marking optical center should be placed at 
proper distance over the dividing line or top of reading portion. 
Figure 175 
A line representing the mechanical axis, or cutting line of the 
lens should then be drawn across the lens as shown in illustration MECHANICAL OPTICS 
173 
(Figure 175) A-B. A line is then drawn at right angle to cutting 
line marking center of wafter as shown C-D, C representing 
optical center of distance lens. 
If the lens is to be a toric cylinder or spherical cylinder instead 
of a meniscus a line denoting the axis of the cylinder is then 
drawn from each edge of the lens, passing through the dot 
representing the optical center as E-F, the line to be drawn at 
correct axis to meet the requirements of prescription. Marks 
must then be placed on lens where the calipering or measuring of 
the thickness of edges will be done, otherwise the lens would 
be prismatic, and the optical center would not coincide with dot 
C. The lens is now ready for blocking and will be ground the 
same as a meniscus or cylinder, as the case may be. 
Kryptoks 
Books of instruction on the grinding of single-piece bifocals 
can be obtained from manufacturers, but it will not be amiss to 
quote from these in this chapter. 
To the Kryptok Sales Company, Inc., we are indebted for the 
following instructions regarding Kryptoks, the fused, one- 
piece bifocals: 
Blocking the Kryptok Blank 
"Much of the breakage in grinding Kryptoks is caused by 
improper blocking of the blanks. It is important to select a lens 
block with a curve near to the curve of the blank. Put the blank 
on a block that will keep the pressure evenly distributed instead 
of on the center of the blank. 
"By using a block with practically the same curve as the blank 
uniform thickness of pitch can be had at all points. The layer 
of pitch should be at least 3 mm. thick. 
"The blank should be heated gradually and never placed 
directly in the flame. The blank and lens block should both be 
heated. The blank should be allowed to cool before grinding, but 
never cooled by dipping in water. 
"Blanks left on the lens block over night may break on ac- 
count of the change in temperature, causing uneven contraction 
of glass and pitch. 
"To take the lens off the block it may be chilled in cold water. 
It should be chilled slowly, however, as sudden changes in tem- 174 
MECHANICAL OPTICS 
perature may crack the Kryptok on account of the difference 
in the expansion of the crown and flint glass. 
Grind and finish the disk side first. 
"Roughing: This operation is to remove the surplus glass. 
The blank should not be roughed down too closely, as deep 
scratches may be made in the lens which cannot be easily re- 
moved. 
"Care should be taken in the roughing not to grind the disk 
side unevenly. By watching closely the process of the grinding 
and tilting the blank as necessary the circle of the disk will be 
preserved. 
"Smoothing: With smoothing emery reduce the diameter of 
the disk to within 2 mm. of its finish size, if a high addition, or 4 
mm. if a low addition. 
Grinding Kryptoks 
OST 
Figure 176 
" Fining; With finishing emery reduce the size of the disk to 
its final diameter. This operation is a very important one and 
care should be taken to obtain the best possible surface with the MECHANICAL OPTICS 
175 
finishing emery. Do not leave too much to the polishing or the 
disk will be wavy and the lens off focus in the reading addition. 
"Practically all the off-focus trouble is caused by insufficient 
fine-grinding and too much polishing. 
"Polishing cloth of uneven weave and thickness causes wavy 
segments and off-focus trouble and we would warn against the 
use of worn cloth of any kind. The right polishing cloth will 
save much loss of labor and spoiled blanks. 
"After finishing the segment side, the lens is removed from the 
block and marked for grinding the second side. 
"First mark a dot at top and center of disk. The blank is then 
placed on the K. S. Co. Protractor (Figure 176), with rough 
side down and disk in proper place. To locate correct position 
of disks, use fine lines below center of protractor, which measure 
1 mm. each, use these for decentering disk, then mark on the 
finishing side the direction in which cylinder is to be ground. 
"The geometrical center of the distance portion should be 
located and dotted and the blank chipped away until the optical 
center is equally distanced from opposite edges of both the long 
and short diameter. To avoid grinding prismatic keep the 
opposite edges of lens a uniform thickness. 
"The following table will show what the actual thickness of 
the disk should be at its center. If the lens is ground thinner 
it immediately grinds into the disk and the blank is ruined. 
"(Diameter of Disk 19 mm.). The following table shows the 
minimum thickness of a finished Kryptok measured at the center 
of the disk. If ground thinner than the figures given, the flint 
disk will be exposed on the second side and the lens destroyed. 
Addition Min. Thickness 
Addition Min. 
Thickness 
.50 
.3 mm. 
or \y2 
points 
2.75 
1.6 mm. 
or 8 points 
1.00 
.6 " 
3 
4 4 
3.00 
1.7 
" 
1.50 
.9 
4X 
4 4 
3.25 
1.9 
9J4 " 
1.75 
1. 
5 
4 4 
3.50 
2. 
10 " 
2.00 
1.1 " 
5X 
4 4 
3.75 
2.1 
2.25 
1.3 
4 4 
4.00 
2.2 
11 " 
2.50 
1.4 
7 
44 
Laying-out and Marking Kryptoks 
"Shopmen have different methods of laying out and marking 
Kryptoks. A very simple method is described below: 176 
MECHANICAL OPTICS 
"First neutralize the distance correction-both the spherical 
and cylinder (if any)-then the reading portion in the piano 
meridian of the cylinder and across the cylinder power. 
"Next dot the axis of the cylinder and optical center. Then 
place small dots around the edge of the reading portion, about 
2 mm. apart and dot the center of the disk. 
"Now the lens should be placed in the Kryptok Protractor. 
"In using this method it is well to draw a circle of 20 mm. on 
the protractor allowing 2 mm. between the 180° meridian line 
and the top of circle. This is spaced off with small dots 1 mm. 
apart on both sides of the 90° meridian. Then in case the disk 
is to be set-in it is a simple matter to place the dot on the center 
of the disk just over the line on the card and in this way ac- 
curately set-in the disk 1 or 2 mm. as the case may be. 
Cutting and Edging Kryptok Lenses 
"The first operation in cutting a Kryptok lens is to locate and 
dot the optical center of both distance and reading portions. 
Figure 177 
Also the axis of the cylinder, if any. Our cardboard protractors 
are especially designed for marking Kryptoks. MECHANICAL OPTICS 
177 
"After locating the optical center of the distance and reading 
portions the lens can then be properly placed in the cutting 
machine. 
"We strongly recommend the use of the Genothalmic Lens 
Cutter (Figure 177) for cutting Kryptok lenses. This machine is 
so designed that the pressure applied in holding the lens is on 
the center and not on the edge to be cut off. This feature will 
prevent much breakage in cutting Kryptoks. 
"The Genothalmic Cutter has the additional feature of uni- 
versal adjustment of sizes without the use of patterns, and the 
tilting diamond, which cuts from the under side of the lens. 
The cutting point is always in sight and the diamond can be 
tilted to always make the cut at right angles to the curve of the 
glass. This reduces breakage and makes a clean cut, so that 
edge grinding is reduced to a minimum. 
Using the Kryptok Protractor 
"First grind and finish segment side. 
"Mark dot at top and center of vertical meridian of segments. 
"Place rough side of blank on protractor (Figure 176) with 
segment in its proper place. Then mark with blue pencil the 
direction in which the cylinder is to be ground. Be sure to cut 
off surplus glass. This will prevent a prismatic effect. (Do not 
fail to decenter if desired.) 
"Extreme care must be used in grinding cylinders at the correct 
axis. 
"Toric Kryptoks: The circle of numbers, as found on the out- 
side of the chart, is to be used for toric prescriptions where the 
cylinders are plus, and must necessarily be transposed to minor 
cylinders, as a cylinder cannot be put on the disk side of a Kryptok 
lens. 
"The inside circle of numbers represents the axis to be used 
for toric prescriptions where cylinders are minus and not to be 
transposed. 
"Flat Kryptoks: The middle circle of figures represents axis 
to be used for prescriptions where cylinders are to go on the 
outside of the lens (7. e., on the side farthest from the eye). In 
this case it is necessary that the segment shall be placed on the 
inside of the lens. 178 
MECHANICAL OPTICS 
"The inside circle of figures represents axis to be used for 
prescriptions where cylinders are to go on the inside of the lens 
(7. e., nearest to the eye) and the segment side to go out farthest 
from the eye." 
Steadfast Bifocals 
Among the distinct types of one-piece bifocals the Steadfast 
represents one, the manufacturers of which supply the following 
instructions: 
"The blanks are supplied finished on the concave or bifocal 
side only, the power for reading being fixed in the original grind- 
ing. The blanks are supplied in all bifocal additions with the 
curve of the distance portion either - 4.50 - 6.00 or - 7.50. 
"To finish Steadfast blanks the usual surface grinding pro- 
cedure is followed, and for spherical lenses the regular spherical 
laps or grinding tools are employed, it simply being necessary 
to grind the power required for distant vision on a rough blank, 
selected with the proper additional power for reading. 
"For piano cylinders the regular toric laps or grinding tools 
are used, those on a 6.00 base curve. Forsphero cylinder com- 
binations a cylinder grinding tool with the proper base curve is 
required and for every different power of sphere there would 
be a series of grinding tools comprising all of the cylindrical 
powers. 
"The practice is to compute the base curve of the cylinder 
grinding tool by adding the spherical element to the base curve 
of the blank and then select a tool with this base curve which will 
have the proper cylindrical value. The concave 6.00 base curve 
blanks are used principally. The 4.50 curves are usually em- 
ployed for strong convex lenses and the 7.50 base curve blanks 
for strong concave. The instructions for finishing Steadfast 
one-piece invisible bifocal lenses are generally the same as those 
used in the finishing of any type of one-piece bifocal." 
Ultex One-Piece Biocals 
Illustrations (Figures 178 a, b, c and d) represent the four 
different styles in which the Ultex One-Piece Bifocal is made. 
The "E" Style Ultex is the lens most used for general purpose 
bifocals. It furnishes glasses perfectly comfortable for the street MECHANICAL OPTICS 
179 
and at the same time meeting every ordinary need for near vision. 
But for those engaged several hours daily in the library reading, 
Figure 178 a 
or at the office working over books or reading correspondence, 
or in the drafting room tracing fine detail, Style "A" Ultex is 
recommended. 
The "B" Special Ultex is not recommended for general use 
owing to the small size of segment. It does not permit of a free 
and natural mode in reading. Once in a while there are cor- 
Figure 178 b 
rections that can be handled to advantage from an optical 
standpoint with Style "B" Special Ultex, but otherwise it is 180 
MECHANICAL OPTICS 
recommended only for an extra pair for dress purposes for the 
extremely fastidious. 
Figure 178 c 
The "L" Style is useful when the lower or reading portion is 
intended to cover a very large field. 
The "A" Style Ultex has a reading portion 19 x 38 mm. in 
Figure 178 d 
size, the "E" Style 16 x 32 mm.; the "L" Stlye 24 x 30 mm.; 
and the "B" Special a reading portion 22.3 mm. round. MECHANICAL OPTICS 
181 
Surface Grinding Ultex to Avoid Prism 
The following instructions, written by L. W. Bugbee, for grind- 
ing Ultex Bifocals were kindly supplied by the manufacturers, 
One-Piece Bifocal Company. 
"There are several methods in use for laying out and grinding 
Ultex blanks. In any case, the first operation consists in locating 
and dotting the required position of the optical center of the 
Figure 179 
distance portion. On this center, as a pivot, swing the reading 
portion in the required amount, say mm- After insetting 
the segment, dot the position to the axis. Now crumb the 
blank to approximate size of the finished lens, allowing enough 
for edging, as shown in Figure 179. 
"Next dot the measuring points a, b, c, d, making the distance 
a e equal to e c, and d e equal to e b. Grind thickness a equal to 
thickness c, and thickness d equal to thickness b, then the optical 
center will be at e as required, even though the lens should be a 
sphero-cylinder oblique axis. Some shopmen make a equal to 
d and c equal to b, which will give the same result with a sphere 
or a cylinder axis 90 or 180, but if cylinder is oblique the latter 
method will displace the optical center from its required position. 
"Some shopmen measure at 1 and 2, also 3 and 4. Of course it 
is necessary to make an allowance for the added thickness of the 
reading segment, and the accompanying tables show the allow- 182 
MECHANICAL OPTICS 
ance to make for 'E' segment 32 mm. diameter and 'A' segment 
38 mm. diameter. 
Addition 
'A' Segment 
'E' Segment 
Allowance 
Allowance 
Millimeters 
Millimeters 
.50 
.1 
.1 
.75 
.2 
.2 
1.00 
.3 
.2 
1.25 
.4 
.3 
1.50 
.5 
.4 
1.75 
.6 
.4 
2.00 
.7 
.5 
2.25 
.8 
.6 
2.50 
.9 
.6 
2.75 
1. 
.7 
3.00 
1.1 
.7 
3.25 
1.2 
.8 
3.50 
1.3 
.9 
3.75 
1.4 
1. 
4.00 
1.5 
1. 
4.25 
1.6 
1.1 
4.50 
1.7 
1.1 
4.75 
1.8 
1.2 
5.00 
1.9 
1.3 
5.25 
2. 
1.3 
5.50 
2.1 
1.4 
5.75 
2.2 
1.5 
6.00 
2.3 
1.5 
"Above allowances are in tenths of a millimeter. Most of the 
gages in common use are graduated in fifths of millimeters, 
which graduations are commonly referred to as points, so that 
in using above tables .1 would equal one-half a point on the gage, 
.4 would be two points, etc. 
"To grind a lens according to this method suppose the ad- 
dition to be + 2.50 'A' style segment. The allowance according 
to the table is .9 mm. and the lens at point 1 (Figure 179) 
will be .9 mm. or points on the gage thicker than at its 
opposite side or point 2, and the optical center will be at point 
e, as required. In other words, if center is required to be at e, MECHANICAL OPTICS 
183 
exactly half-way between 1 and 2, and segment is 'A' style 2.50 
add, and point 2 measures 1.5 mm., then point 1 would measure 
1.5 + .9, or 2.4 mm., or 14 points on the thickness gage. Of 
course point 3 should measure the same as point 4. 
By this method, after the shopman becomes accustomed to it 
the grinding of an Ultex blank becomes as simple as that of a 
single-vision lens. 
If a prism is to be ground upon the blank, mark the base apex 
line and measure the prism on that line at the opposite edges of 
the lens. To find the difference in thickness for a given amount 
of prism, measure the total diameter of the lens on the base apex 
line. Multiply this diameter by .02. The result will be the 
required difference in thickness for 1 prism diopter or 1 degree, 
as it is often called. Multiply this value by the prism diopter 
required. CHAPTER XII 
Soldering and Repairing 
SOLDERING is a part of mechanical optics not attempted 
by many opticians. While it is not very profitable, per- 
haps, it must be done occasionally to satisfy a customer. 
It also frequently happens that it must be done in a hurry, as 
there is no time to send it away. There are many jobs that 
opticians are called upon to do in the way of small repairs, and 
one thing that is necessary is to have proper equipment and the 
work then will not be difficult. 
Gas is very essential to do good soldering, although it can be 
done with an alcohol lamp. Alcohol is very unsatisfactory, for 
the reason that it is impossible to obtain a large flame and 
therefore there is not heat enough. 
There are several ways of arranging the gas, and possibly 
the simplest is to use an ordinary swinging gas bracket and re- 
move the lava tip. The size of the flame can then be regulated 
and an ordinary blowpipe used (Figure 180). There are two 
styles, one having a bulb, forming a sort of reservoir for air. 
Figure 180 
Either will do, however. One of the most efficient blowpipes 
is the one illustrated in figure 181. It has a substantial iron 
base enabling the operator to use two hands when soldering, or if 
work is pinned to a block the blowpipe can be handled so as to 
place the flame where desired. This is a compound blowpipe, 
therefore the gas and air can easily be adjusted to the proper 
composition to produce a needle flame. 
There are many styles of soldering blocks, such as charcoal, 
asbestos and numberless patented ones. Webster's soldering 
184 MECHANICAL OPTICS 
185 
block (Figure 182) is the best and this, with a charcoal block, is 
all that is necessary. A borax slate (Figure 183) will be needed 
and this is a slate having a concavity in which borax and water 
are mixed. The borax is prepared in tinfoil and can be used in the 
wrapper, so that it does not soil the fingers. In using put a few 
drops of water on the slate and rub the borax around a few times 
Figure 181 
until the water looks milky. It is then applied with a small 
camel's hair brush. Two jars will be required, one containing 
a solution of one part sulphuric acid and two parts water. After 
the metal is heated in soldering it is blackened and if dipped in 
the acid, while hot, this is removed. It is then rinsed in plain 
water. 
These few articles are all that is required for the soldering, 
with the exception of solder. This can be obtained in several 
forms. The most common is silver, or hard solder. It is sold on 
flat strips, which can be cut with scissors into small bits, or it 
can be had all cut as small as desired. 
Although silver solder will answer all purposes a great many 
use gold solder. This can be obtained in all karats and is usually 
claimed to be "easy flowing." A spool of soft solder is handy to 186 
MECHANICAL OPTICS 
have around, but it should never be used in repairing spectacles. 
For .tools one will need all the ordinary pliers, such as flat, 
round and snipe-nose; also the hollow-chop and cutting pliers. It 
Figrire 182 
will also be necessary to have two or three pairs of tweezers; one 
should be a good pair for picking up screws, etc., but the others 
can be cheap ones. The latter should have a slide so that they 
Figure 183 
can be used to hold articles while soldering. As it is necessary 
to blow the flame on the points, it draws the temper so that they MECHANICAL OPTICS 
187 
are of no use for any other purpose. A pin vise should be added 
for holding wire, etc. This is especially handy when filing down 
wire for rivets and things of that kind. A small vise should be 
screwed to the bench and a small anvil that stands on the bench 
will be found very convenient. A good assortment of screw- 
Figure 184 
drivers should be selected: a good, substantial one with a wood 
handle and two or three others with different size blades and with 
swivel tops. A spectacle and eyeglass stake (Figure 184) is very 
good to hold the endpieces of frames when extracting old screws. 
This is made with a wood base to stand on the bench, or it can 
be had for a vise. There are also a number of screw extractors 
on the market, but one of the spring punches fitted with a screw 
extractor is all right (Figure 185). A couple of small hammers 
will be needed, and have one with a brass head. This will not 
mar or dent some of the softer metals as easily as steel. 
A good assortment of files is required-rat-tail in one or 
two sizes; a flat one four or five inches long and five-eighths 
of an inch wide; a three and a half or four-inch half round; a 
square file, not too large; also a screw head file. For the cut, 
do not have them too coarse. If you have a good assortment 
have them range from Nos. 2 to 6; No. 4, however, is about the 
medium cut. It is well to have all these fitted with handles, as 
better work can be done. These can be obtained in assorted 
sizes for about five cents each. A burnisher is a good tool to 
have, but not too large a one. 
Buff sticks will be needed; also crocus and emery sticks. 
For buffing the ordinary felt stick is all right, but for smoothing 188 
MECHANICAL OPTICS 
and finishing for the buffing a leather stick is required. These 
can be bought with a kind of rawhide, quite thick; the stick 
is also very long. This should be cut down so that it is fairly 
thin; i. e., both the stick and the leather. This will give it a 
Figure 185 
little spring and also allow it to be used in small places, such as 
under the shank of the bridge. The material used with this is 
Tripoli; or pulverized pumice, mixed with oil, will answer. There 
is, however, on the market now a prepared form of Tripoli called 
"Cut Quick." This is in more convenient form. 
These sticks are used mostly, however, if one has no power. 
With power, wheels can be obtained to do most of this work 
more easily. There are certain jobs, however, where these 
sticks will be found very convenient. A buff head should be 
large enough to be substantial, so that it will run true; also that 
the bearings will not wear quickly. If it is too light, the shaft 
will spring easily also. Another point is that, if the bearings are 
not good, it cannot be run at a very high speed without rattling, 
Figure 186 
and this is, of course, very objectionable. It should be run at a 
speed of from two to three thousand and, for all kinds of work 
with the exception of finishing the ends of screws in frameless MECHANICAL OPTICS 
189 
work, it should run toward you. This is a point that some 
workmen do not understand. They have outfits with one buff 
head and this is, of course, fitted to run toward you for buffing, 
etc. Now a screw finisher must run left-handed, or from you, 
because if it did not it would turn the screw out instead of round- 
ing it. On the other hand, if you run the spindle the other way, 
or from you, the screw finisher will be running the right way, 
Figure 187 
but the taper will be threaded the wrong way, so that the wheels 
will turn off instead of tightening. 
It is also the same with the chuck and nut. These will loosen 
so that nothing will stay in them. The only way to overcome 
this is to have a separate head for the screw finisher. If one 
does not have power these heads can be run very successfully 
by foot pow'er. The foot wheel is better than the treadle, as 
more power and speed can be obtained. There are outfits all 
complete, however, with a zinc-lined box and treadle all belted 
if desired. By screwing an ordinary buff head to a bench, then 
placing the foot wheel beneath, a very satisfactory arrange- 
ment is devised. 
For wheels, get a number of stiff brushes (Figure 186), having 
one each of one, two, three and four rows. These are used for 
cutting, as well as cleaning. Then a felt buff wheel from three 
to four inches diameter (Figure 187), a cotton wheel about three 
inches diameter and also a rag wheel six to eight inches diameter 
(Figure 188). Rag wheels come thin, so three to four will be re- 
quired. These should be placed together, with cardboard washers 190 
MECHANICAL OPTICS 
on either side, about two inches diameter, in place on the buff 
next to chuck and screw the nut up tightly. With this kind of 
wheel better results can be obtained in fine polishing, such as 
putting on the final finish on a frame. Care must be used in 
polishing on this wheel, however, as temples and such things 
catch very easily if not held right. 
For other small articles we would suggest a hand or jeweler's 
brush; a roll of binding wire-i. e., soft iron wire, very small, for 
holding parts together while soldering; a stick of polishing rouge 
•-get the kind wrapped up in tin foils so that you need not get 
any more on the fingers than possible; a solder burr for burring 
out the eye wire after solderng; a temple burr for end pieces; 
a bottle of soldering fluid. 
Figtire 188 
The completeness of the outfit depends, however, on the extent 
to which you intend to go into repairing. Years ago it was neces- 
sary to be fitted to do anything, from a simple solder to making 
a frame complete, but today it is entirely different: the goods 
cost less, are made better and a greater assortment of bridges 
can be kept in stock Consequently, when an old frame is 
brought in for repairs it is often cheaper to give the customer a 
new one, or a new part, rather than attempt to repair the frame. MECHANICAL OPTICS 
191 
Now as to the first step in preparing the break to be soldered. 
We will take, for instance, a break in the eye wire. First take a 
fine file-a half-round is the best, as it is pointed and fairly thin. 
It should be very fine; No. 6 is the best. With this, file lightly 
over both sides, or, in other words, the parts to be exposed to 
the flame. Then place it on the soldering block and secure it 
with the clamps, so the ends just come together. Now mix the 
borax in the slate, first putting a few drops of water in the bot- 
tom, and then rub the borax around a few times, until you have 
obtained a milky fluid. Then, with a camel's hair brush place a 
little on the parts to be soldered and apply a small piece of solder 
on one side of the break and you are ready to apply the heat. 
Turn on the gas so the flame will be about two inches high. 
If you are using the ordinary gas bracket, the tip having 
been removed, swing it directly in front of you and hold the blow- 
pipe in the right hand. The soldering block hold in the left, 
at the side of the flame. Now place the blowpipe side of the 
flame close to the outlet and blow a small, steady blue flame 
at the break (Figure 189). This flame should be like a needle 
point, holding the work so the point that touches the frame will 
be about inch in thickness. Heat the part on which the solder 
is placed, and as soon as the solder begins to flow throw the 
heat on the opposite side of the break, thus drawing the solder 
with the heat. 
If the work is held in the hands, the borax is placed on both 
sides and the solder on one side, as before. Apply the heat 
to the side on which the solder is placed, separating the parts 
until the solder is ready to flow. As soon as the solder starts 
to flow, dip both ends of the break in the borax and apply the 
heat on the opposite side, as stated above. 
The cause of melting is blowing too much heat, either with 
too large a flame or blowing too long. Remember that it re- 
quires but a very small point of flame and little heat, but properly 
placed. If a blowpipe is used as described in a preceding para- 
graph, it will be found much more convenient, as it can be held 
in any position. The soldering block can be held in the left 
hand, or laid on the bench, and the blowpipe in the right and 
the flame directed as desired. As soon as the solder flows and 
the eye wire seems to be connected, blow the flame all over 
Preparation of the Break 192 
MECHANICAL OPTICS 
the eye wire adjacent to the break for a second, to warm it, and 
then plunge it into the acid, and rinse in plain water. This 
will remove all the black and leave the frame in good condition. 
The first melting of the solder is the best and makes the strongest 
and neatest job. 
If for any reason the parts did not unite, it will be necessary 
to start all over from the beginning. Do not attempt to add more 
solder and keep blowing at it, for this may cause the frame to 
melt. File the parts again, put on more borax and start again. 
Figure 189 
Smoothing and Polishing 
After the eye wire is soldered satisfactorily it will be acces- 
sary to file the break a little to smooth it, using, of course, the 
fine, or No. 6, cut. Then take the leather stick, referred to 
previously, and with tripoli, pumice and oil or "Cut Quick," 
which is the prepared cutting material, go over the parts until 
all traces of file marks are removed. If power is used, the brush 
is the proper wheel and the cutting material is rubbed on by 
holding the stick against the wheel. Then with a solder burr, 
especially constructed for eye wire, burr out the inside of the 
eye wire. This little tool is used in the buff head and run at a 
fairly high speed, say about 2000 revolutions per minute. 
Polishing the Repaired Job 
You are now ready to polish. For this work use the cotton 
buff and this, of course, runs toward you. Rub plenty of the 
stick rouge into it while it is running. Now hold the frame MECHANICAL OPTICS 
193 
tightly in both hands, with the eyes perpendicular, and the eye 
that has been soldered downward. In this manner there is no 
way for the frame to catch, provided you keep your mind on it. 
You will soon learn to protect the parts that are liable to catch 
with the hands, such as the endpieces and bridge. In all repair- 
ing it is best to remove the te mples, as they are very liable to get 
in the way of the flame or catch in the wheel when buffing. 
After the frame is polished in good shape it should be washed 
with hot water and soap to remove all the dirt and rouge that 
collects in the joints. When replacing the lens it may be neces- 
sary to reduce it a little, as the eye is likely to be made a little 
smaller when soldered. 
Almost all solder jobs can be done without a soldering block 
if one is experienced, but quite a little practice is required. In 
this way the parts are held together with soft iron wire, called 
binding wire. 
Bridges can be obtained all ready to solder to the frame; 
that is, they are bent and grooved for the eye wire. The most 
common method today, however, is to use what is called unbent 
bridges. These consist of the bridge stock in assorted lengths, 
tapered about the right size and thickness for the shank. In 
using this stock the principal feature lies in judging the length 
required. The medium sizes require 2y% to 2% inches. First 
bend the crest or arch; this can be done over the handle of a tool 
or any round piece of wood. It is better to do it in this way 
than to attempt it with the hollow chop or periscopic pliers, as 
these make dents which must be taken out afterward. In gold- 
filled this is impossible, as the gold will be finished too thin. 
After the arch is formed, bend up the shanks. These may 
possibly be too long and should be cut off, allowing stock enough, 
of course, for the feet. A frameless bridge can be soldered 
on straight, and if preferred the frame can be made this way 
also. This style, when made at the factory, however, has a de- 
pression pressed in the eye wire to make a blind joint. In repair 
work this is not attempted. 
The bending of the feet can best be done in a vise, as the 
stock must be turned edgewise, this being quite difficult. After 
the feet are bent they should be grooved with a float file; that is, 
one with rounded edges, just the right size for the eye wire. 194 
MECHANICAL OPTICS 
You are now ready to solder it to the frame. First prepare the 
eye wire by filing it lightly, the same as for any solder. It can 
then be placed on the soldering block, clamped in and the solder 
placed on the top of the foot of the bridge (Figure 190). This 
operation can be done better in the hand, however. The eye 
wire can be held by the joint in the left hand, and the bridge 
in a pair of tweezers in the right hand. By having a slide on the 
tweezers, as before described, it is much easier. If the blowpipe 
is arranged so that it can stand on the bench, both hands will be 
free to work with. 
Another method is to use binding wire to hold the parts in 
position. In soldering the procedure is the same as before, plac- 
Figure 190 
ing the borax and solder on the parts and heating them; as 
soon as the solder begins to flow, throw the heat in the direction 
you wish the solder to flow. Just enough solder must be used so 
that it will flow freely in the groove in the bridge, but it is un- 
necessary to have so much that it will flow outside all over the 
joint. Care must also be used to see that the bridge is in the 
center of the eye wire. When one eye is soldered, the other can 
be put on the same way. The frame can then be lined up and 
the bridge bent to dimensions. 
At this point it is well to see that the crest is the desired 
angle. If not, it can be formed with a pair of crest-angling 
pliers. If one does not have this tool the angle must be made 
first, while the stock is straight. For all saddle bridges, except MECHANICAL OPTICS 
195 
the lowest ones, such as no height, the natural bend of the stock 
produces about 45°. "C" bridges, however, must be bent first. 
This operation is done by tilting the stock edgewise a little. This 
can be done by holding both ends of the stock securely with 
pliers and bending it over the edge of the bench pin. After the 
dimensions are made right any superfluous solder can be filed 
off with a fine file and the bridge finished with the buff stick and 
Cut Quick. 
With power this can be done much better and quicker. For 
this work use the brush wheel, putting on plenty of the cutting 
material. This wheel reaches in under the shank and smooths 
up the foot very nicely. After smoothing up in good shape it 
can be polished in the regular way, being very careful, however, 
to hold the frame tightly, so that the shank will not catch in wheel. 
Breaks in the Shank 
One of the most common breaks is in the shank, at the turn. 
Formerly this was considered an unsatisfactory job, as, in order 
to make it strong, quite a little solder must be left in the turn. 
This was objectionable and yet, if finished in good shape, it made 
a very weak joint, as considerable strain comes at this point. 
Now this is a very common job, however, but the practice 
generally is to leave plenty of solder; in fact, sometimes it is 
pretty well filled up. For this operation it is best to use binding 
wire, so as to bring the parts together in the right position. The 
parts must be prepared in the same manner as previously ex- 
plained and the process is the same. 
First prepare the eye wire by filing, then groove out the end- 
piece, unless possibly it has just pulled out. Most breaks of this 
kind, however, are next to the endpiece, so that a part of the 
eye wire is in the joint. When doing a job of this kind, be sure 
and remove the temple and screw and separate the joints. Do 
not, under any circumstances, attempt to solder an endpiece 
on the eye wire, having them screwed together, for if you do you 
will find both endpieces soldered together and it will be like one 
mass, and it can never be taken apart. For this operation the 
endpiece can be bound to the eye wire with binding wire, and in 
this case wind it around the eye wire several times, quite a little 
distance back from the joint, and insert the other end through 
the screw hole and place it on the end of the eye wire. Do not 196 
MECHANICAL OPTICS 
attempt to bind it so that the eye wire comes near the solder. A 
simple way to do this work is to hold the frame in one hand and 
hold the joint on an old file, by inserting it in the screw hole. Do 
not use much solder in this operation, as it requires but the 
smallest piece to hold securely. After the joint is soldered it 
will be necessary to burr out the eye wire and possibly reduce 
the lens a trifle. 
This material is worked the same as gold, except that one must 
bear in mind that there is only a very thin coating of gold over 
the base metal. One stroke of the file will cut through it, con- 
sequently the work must be done in such a manner that no filing 
is necessary. The great feature in handling gold-filled is to use 
the smallest amount of solder possible. In this w'ay it flow's only 
into the break or betw'een the joints. At the factory this work is 
usually covered with a coating of boracic acid to prevent discolor- 
ing. This is a pow'der and by wetting the frame and placing it in 
the pow der enough will stick to it. The polishing should be done 
as quickly as possible, as the metal will not stand too much. 
Temples are very unsatisfactory to repair and only in cases of 
emergency should it be attempted. They are made in gold by 
drawing, and in gold-filled by swaging. This process gives them 
the temper and spring. As soon as heat is applied to them they 
are annealed, and consequently are very soft. The best way to 
repair them is to use a ferrule. These are small pieces of tubing 
just the right size to slip over the broken ends, and as the base 
metal is solder it is only necessary to apply the heat and a good 
joint is obtained. This method leaves a bunch on the temple, 
but it is fairly strong. 
Spring stock must have considerable temper and spring, con- 
sequently w hen heat is applied it is practically useless. When a 
shank of this material is broken near the end, or screw hole, a 
new hole can be punched, and although it makes it somew'hat 
shorter it is very satisfactory. This is the only way any kind 
of a spring can be repaired, but even this method hardly pays, 
as springs can be obtained at a very low price. 
Soldering Steel 
Steel goods are fast disappearing, although the better class of 
trade still call for them occasionally, preferring to wear them 
instead of'gold. It seldom, if ever, pays to repair them, unless MECHANICAL OPTICS 
197 
possibly it is to solder on an endpiece. Steel is handled in 
much the same manner as gold, except, of course, silver solder is 
always used. The parts to be soldered are first filed bright, so 
that all traces of rust or foreign substances are removed. The 
parts are then dipped in borax and the heat and solder applied 
as described previously. This metal must be heated to a greater 
temperature; in other words, red hot. It is then plunged in the 
acid and this removes the black and also restores the temper to 
a certain extent. In finishing, the surplus solder is filed away and 
smoothed. The frame then should be rubbed with fine emery 
Figure 191 
cloth to leave the steel bright. If it is to be nickeled, no more 
finishing is required. If the finish is bronze or blue it must be 
refinished to match as nearly as possible the old finish. 
At the factory the process is simplified by using hot sand. This 
produces an even color of any shade desired. For small shops it 
is hardly practical to attempt this method, as it is so seldom neces- 
sary. The easiest way is to use an alcohol lamp (gas will not 
answer, as the regular flame will blacken the work and a Bunsen 
burner has too much pressure). When using the ordinary jewelers' 
alcohol lamp the wick should be pressed down so that a very 
small flame is obtained. The frame is then held in the left hand 
and the lamp in the right. Take the work to a place where good 
daylight can be obtained. Neither artificial light nor poor day- 
light will give proper illumination. Now apply the flame directly 
to the metal (Figure 191) and then remove it quickly so that it is 
heated but a very little. Repeat the operation several times, 
being careful to heat the frame but a second at a time. The 198 
MECHANICAL OPTICS 
color will be observed after the flame is removed and it will be 
noticed that it changes considerably in a second or two. Bronze 
will appear first, then blue, and, if you are not careful, you will 
burn it and bright spots will appear. As soon as each spot is the 
desired color move the flame along. This is one of the most 
difficult jobs in repairing, and to produce an even color re- 
quires considerable practice. If the color gets too deep refinish 
the frame with emery cloth and try again. 
Cutting a Frame 
We are frequently called upon to cut a frame to fit old lenses. 
In this work all four endpieces, if it is a spectacle, must be 
moved. This requires four solders, so it is easily seen why this 
work is expensive. If one endpiece on each side is moved the 
frame will be out of shape. Take one endpiece at a time, blow 
the heat on to remove it, then refinish it and set it back on the 
eye wire slightly. The end that overlaps can then be filed off 
square. There are lens washers now on the market which are 
used extensively for this purpose and these are very satisfactory 
(Figure 192). If they are not at hand, tinfoils, or, better still, tea 
Figure 192 
lead, can be inserted in the eye wire to fill the open space. If 
the lens is only a very little loose, the endpieces can be filed 
slightly, but this method reduces the thickness so that it weakens 
the joints considerably. It is always preferable to fit new lenses 
if possible and this can frequently be done without the customer's 
knowledge. 
Screws for all standard frames can be obtained cheaply so that 
it is well to have a good supply on hand. If one has taps for 
these threads they can be used to replace odd makes by simply 
tapping new threads (Figure 193). The great difficulty is in 
removing the old screw, however. For most jobs the screw 
extractor, referred to on page 187, will suffice. This can be used MECHANICAL OPTICS 
199 
for endpiece screws if the head is not too far gone. If the head 
has been turned off completely it will probably be necessary to 
drill it out. Small twist drills, such as can be purchased in any 
hardware store, are used for this purpose. After the screw is 
drilled out the hole must be tapped and the new screw inserted. 
If the ends protrude they should be filed off carefully and the end- 
piece polished. Stud screws, or screws that project, can fre- 
Figure 193 
quently be removed successfully by filing a slot in the head with 
a screw-head file. By then placing the frame in a stake or against 
a solid square surface (if a stud screw hold it with a pair of stud 
or round-nose pliers) and by using a wooden handle screwdriver 
it can be turned gradually. 
Fingerpiece mountings are used so extensively at the present 
time that it is well to be prepared to fit these while the customer 
waits. The factories will possibly replace these without charge 
but it seems absurd to require a customer to go without their 
glasses for from one to three days when these can be inserted in a 
few moments. These springs come in rights and lefts and with 
Figure 194 
two, three and four coils. The best mountings have four coils, 
as this type produces an easier pressure and is less liable to break. 
They are placed on the screw with the top ends over the inside 
edge of the straps, the coil being wound from left to right on the 
right side and from right to left on the left side (Figure 194). The 
lower end is turned under the guard. After both ends are bent 
so they are secure they can be cut off fairly close, as will be 
readily seen. 200 
MECHANICAL OPTICS 
In case the arms of the guards are broken on any style of finger- 
piece mounting it is far better to replace them with an entirely 
new guard, as soldering is always unsatisfactory and is far from 
neat in appearance. 
It is frequently necessary to fit new zylonite to guards, although 
it hardly pays, except on gold. There are a great many styles 
where the zylonite is peculiar in shape, and at the factories these 
are punched out with dies. When one is called upon to furnish 
one of the irregular shapes the best that can be done is to cut it 
with shears or a knife as near as possible. This material can be 
smoothed and shaped with a file so that it is possible to do a very 
good job. Zylonite can be obtained in any form, such as plain 
sheets, plain strips or in pieces ready to be fitted. The corruga- 
tions are pressed in at the factories, so it can be obtained in this 
form if preferred. It is well to have a small assortment, how- 
ever, so that all sizes can be obtained. 
Small wire or even common pins are used to secure it to the 
guard, but rivets, made especially for this purpose should be kept 
on hand. The guard is already punched and small holes should 
be made in the zylonite just large enough to force the pins 
through. These are driven through from the back and are then 
cut off quite close and with a light hammer riveted gently. If 
the holes are just the right size it requires but a few taps. If 
plain zylonite is used, it will be necessary to corrugate it with a 
screw-head file, making the cuts as small and as regular as pos- 
sible. Scraps of shell can also be used for this purpose. 
Corks have to be fitted quite frequently, although many times, 
if they are only soiled, they can be washed with soap and water. 
1 his is the method employed when guards are slightly soiled from 
lying in stock. If they are quite bad and not worn down they can 
be sandpapered to look as good as new. For this purpose No. 00 
sandpaper is used; any grade coarser than that will rough them. 
Cork can be obtained in the rough, cut the light size, or it can be 
had all rounded and finished with groove, ready to fit into the 
guard. The difference in price is so slight that it is better to use 
the finished. 
The old cork should be removed and the edges of the guard 
lifted with a burnisher or a knife just enough so that the cork will 
slide into the guard easily. The edges can then be burnished down MECHANICAL OPTICS 
201 
so that it will hold securely. The end can then be trimmed oft' 
and if the guard is then too thick it can be sandpapered down. 
Some styles, such as No. 0 Anchor, have a round disk which 
must be riveted and for any of these styles the disks can be 
ordered, cut to shape. The regular styles of guards in nickel and 
gold-filled are low-priced, so that it is often cheaper to fit new ones 
than the charge made to cover the cost. 
Dowels, or rivets, as they are more generally called, have to be 
fitted to spectacle endpieces. In gold-filled they loosen and fall 
out easily. In gold they wear so that the temples are loose and 
must be replaced. The only satisfactory way to tighten old gold 
temples is to fit new dowrels, but for quick repairs spec washers 
can be obtained. Two, three, or as many as are necessary are 
placed on the dowel to fill up the space in the joint and the end- 
piece screw'ed together. 
Dowels can be obtained in the different sizes ready for use. 
They are tapered and left long so that they will fit any joint. 
\\ hen fitting these, it is only necessary to drive out the old one 
and insert the new'. Place the joint on an anvil or vise so that the 
small end of the dowel can be driven into a hole and with a light 
hammer tap it lightly a few' times and then try the temple. If 
it is not fairly tight, drive it in a little farther. If it is still loose 
drive it out again and with a small broach ream out the hole a 
little. The dowel can then be inserted and driven in slightly. 
Do not have the temple fit too snug but see that it w'orks fairly 
stiff. If it works too freely it may be too loose after the ends of 
the dow'el are finished off and it w ill then be necessary to do the 
wrork all over again. When the dowel is in place the ends should 
be cut off closely and finished first with a fine file and then pol- 
ished on the buff wrheel. 
If the finished dowels are not available, steel wire can be used; 
a size a little larger than the hole should be selected and a piece 
about a half an inch long cut off. This should be placed in a pin 
vise and filed to a slight taper. It can be done on the bench pin, 
revolving it with the left hand and filing with the right. 
In gold-filled, the dowels are usually rounded on the ends and 
for this metal the finished dowels for the particular grade that 
ycu are using should be kept in stock. A regular dowel can be 
used, however, and the ends rounded with a screw finisher. As 
gold-filled is a softer metal the temples are fitted more loosely and 202 
MECHANICAL OPTICS 
the joint brought together tightly. This may not seem practical, 
but it is the only way the dowels can be made to hold in the end- 
pieces. If they are fitted the same as gold the temple will work 
them loose. In some grades of gold-filled the flush dowel is used 
and the largest end is punched slightly with a very small punch 
(Figure 195). This is supposed to spread the head slightly to hold 
it in place. 
There are a number of small jobs that one is called upon to 
do that are interesting if time is of no account, such as drawing 
out gold temples, or balling them. To ball a gold temple it is 
Figure 195 
only necessary to file the end slightly, to clean it and then dip it 
in the borax and apply the heat. This is very convenient when it 
is necessary to shorten temples. Gold-filled cannot be done in 
this manner but requires a gold ball soldered on the end. This is a 
little more difficult and is not attempted unless necessary. 
The fitting of lorgnette springs is now quite common and this 
is a job that requires quite a little ingenuity. The springs can be 
obtained quite reasonably and if one has the time and cares to 
experiment with them, a little practice will enable him to do as 
good a job as can be done at the factory. When fitting the center 
springs to plated styles it is necessary to have them replated. 
Platers when doing this work often use acid, which destroys the 
spring, so they should be cautioned regarding this point. The 
older styles, such as heirlooms, are very difficult to repair and 
should not_be attempted. 
The inspection of a completed prescription calls for more than 
a mere glance. After the workman in a large shop has mounted 
a spectacle or eyeglass he does not attempt to straighten or "true 
Inspecting a Prescription MECHANICAL OPTICS 
203 
it up," but it is passed along to a truer or inspector. If the shop 
is large enough for a truer, the work will pass through his hands, 
and his part is to straighten and line up, or, in other words, put 
the glasses in shape. It is then passed along to the inspectors, 
whose part of the work is to see that the prescription is filled cor- 
rectly and to put on the finishing touches. Even though the truer 
may be an expert workman, it is perhaps impossible for him to 
allow each job the necessary time and, consequently, a temple 
must be curved a little more or some other little point must 
receive the attention of the inspector. In many of the large shops 
this work is done entirely by the inspectors, and with this method 
the best results are always obtained. 
An inspector must necessarily be an expert; he must not only 
be familiar with mounting in all its branches, but he must have a 
thorough knowledge of lenses and thoroughly understand the 
different prescriptions. The reading of a prescription intelligently 
often requires quite a little study. In the first place, the writing 
Figure 196 
is not always legible, and then again, two different meanings can 
often be interpreted. The work of inspecting should be carried 
through systematically, for the reason that if the time is spent 
truing up the mounting and then a lens is found to be incorrect, 
the work was done for nothing. Each time a lens is fitted the 
mounting is thrown out of adjustment, even if only a very little. 204 
MECHANICAL OPTICS 
The Lenses 
The first step is to neutralize the lenses; this should be done 
with a spherical test case. Cylnders should never be used, as 
they are more awkward and necessitate the lining up of the lenses 
first. With sphericals the spectacle can be held at the proper 
angle, or as near as possible, and the lens neutralized in this 
direction. It can then be turned at right angles and the opposite 
meridian tested. If preferred, the first test lens can be held in 
position and another spherical lens, with the same focus as the 
cylinder calls for, held over it. The better way, however, is to 
use one for the spherical, then remove it, and place one in posi- 
tion for the other meridian, having the combined strength of the 
spherical and the cylinder. 
Figure 197 
For example, the lens to be neutralized is + 1 O + 50 ax. 
90°. First take a - 1 spherical and test the vertical meridian; 
then remove this and select a - 1.50 to neutralize the opposite 
meridian. In this operation no attention is given to the axis, 
except that the glasses should be held so that the axis is approxi- 
mately perpendicular. The lens measure is used by many for this 
work, and if it is to be relied on, must be watched constantly, MECHANICAL OPTICS 
205 
to see that it does not get out of adjustment. With constant use 
the three points gradually wear, so that new points need to be 
put on occasionally. This work must be done at the factory, but 
by testing it at intervals on a plane prism and adjusting the center 
pin it can be kept in condition. 
The next step is to locate the axis and mark it. This should 
be done on a machine if possible, but if one does not have one, 
a straight line drawn on a card will answer for medium and 
strong-power lenses, and one on the wall, at a distance, for the 
weak power. (For instruction in this work see Chapter IV, 
"Marking Lenses.'' This explains the breaking of the line and 
the drawings show how the axis can be located.) Three dots are 
then placed on the lens, one being the center (Figure 196). The 
Figtire 198 
spectacles are then laid on an axis chart (Figure 197), with the 
glass screws on the 180° line, and the two dots on the extreme 
edge will locate the axis. 
A machine for this purpose is the Standard centering ma- 
chine (Figure 198). This is equipped with a holding device in 
which the spectacles are placed. An axis dial on the back can be 
operated so that it will revolve. When the spectacles are placed 206 
MECHANICAL OPTICS 
in position the line will appear broken (Figure 199); the dial is 
then revolved until the lines are continuous (Figure 200). The 
axis is then read from the dial. For example, if the lens was axis 
Figure 199 
45° and the machine happened to be set at 90°, the lines would 
appear as in Figure 199. 
Figlire 200 
The advantage of a machine over the method of holding the 
glasses by hand is that the spectacles are held rigidly and the MECHANICAL OPTICS 
207 
axis is read quickly from the dial; it is unnecessary to dot the 
lenses and then place them on the an axis chart. 
There has been a machine in use by a well-known retail house 
by which the axis is located electrically. It is known as the 
Lloyd axonometer (Figure 201) and is now on the market. With 
this machine the axis and focus can be located both quickly and 
accurately. 
There is also the Wellsworth Lensometer, an instrument that 
accurately tests lenses as to focus and axis according to their 
effective power. (Figure 202.) 
The axis having been located, the next step is to inspect the 
size and shape of the lenses. If it is a repair job, the new lens 
must match the old lens on the opposite side. The size is meas- 
ured for length and width with a rule, and this should be read in 
millimeters. If the prescription calls for a pair of 00 eye lenses 
the measurements will be 40 mm. long by 31 mm. wide. Although 
it is perhaps necessary to measure both lenses, an experienced 
Figure 201 
eye will readily detect a difference in size of the the two lenses. 
This can be noticed as the shapes of the two are compared. If 
the lenses are ground in an automatic machine they must, of 
course, be alike, but when ground by hand it is a very easy matter 
to get one different from the other. This is especially true in 
repair jobs, where a new lens is ground to match an old one. It is 
possible to grind two lenses with exactly the same measurements 
and still have a decidedly different oval (Figure 203). This part of 208 
MECHANICAL OPTICS 
the work is where the expert inspector counts, and many jobs are 
rejected for this reason. With a little practice the shape can 
readily be compared and at the same time the difference in the 
size will be noted. 
Figure 202 
The lenses having been found to be the correct size and shape 
the inspector then notices if the bevel on the edge is about right. 
Figure 203 
If too little bevel has been placed on the edges, they will chip 
easily, therefore the job must be returned to the grinder. For MECHANICAL OPTICS 
209 
this operation it will be necessary to take the lenses out, and 
they, of course, must be remounted. Here it will be noted that 
if the spectacle had been trued up the time was wasted, and the 
truing must be done again. If the bevel is too deep, or an 
attempt to take out a chip has been made, thereby leaving the 
bevel uneven, a new lens must be ground. A lens that has been 
beveled too much will spoil the appearance of any pair of glasses, 
as this gives the lens the appearance of having a white edge. A 
good illustration of this is a pair of bevel edge or frame lenses 
mounted up as a frameless spectacle. 
When inspecting the bevel the edges must be watched care- 
fully for chips. These are often minute, but when exposed to an 
electric light will sometimes sparkle like a diamond. In grinding 
lenses by machine, the edges should come out perfectly clear and 
as sharp as a knife edge, provided the grindstone is in good con- 
dition. Many stones are neglected, however, and with constant 
grinding over one part of the surface slight ridges will appear 
at the edges, and if these are not turned out the lenses will chip 
the least bit, perhaps not enough so they cannot be taken out in 
beveling. A workman, in handling quantities of lenses, is very 
apt to overlook some of these and, when located in the finished 
job, the lenses must be taken out of the mountings and the chips 
run out. Occasionally a chip on the top or bottom of the lens 
can be taken out without removing the lenses from the mounting, 
but care must be used not to strike the metal on the stone. A 
long, quick sweep of the lens is required; do not attempt a short 
one, as this will give the lens an irregular appearance. Make it 
as long as possible and, of course, not very deep. 
If one has plenty of time the surface can be examined at this 
point for scratches, but as it necessitates cleaning the lens thor- 
oughly the inspector usually waits until the mounting has been 
trued up. He can then wipe the lenses once and give the job the 
finishing touches. If he cleans the lenses at this point he will then 
soil them in the truing, and this makes an extra operation. This 
may seem trifling to many opticians, but every move counts when 
examining several hundred jobs in a large shop. If the truing has 
been done and a scratch found, a new lens will be required. You 
may say that the truing must be done again; that is true, but the 
inspector takes this chance. 
One may make the suggestion that all this may take consider- 210 
MECHANICAL OPTICS 
able time and that the prescription house does not give them 
this attention. This is not so, as every job passes through this 
same rigid examination, but the operation to a trained eye re- 
quires a very few seconds. Practice will enable any optician to 
be very critical and many are expert in this work. The larger 
majority, however, we are afraid, do not inspect their jobs at all, 
but depend entiiely on the prescription houses. Although the 
best houses rarely make a mistake, it is by all means wrong to 
place a pair of glasses on a patient without giving them the 
proper inspection. 
The lenses having been found to be correct, attention is then 
directed to the frame or mounting. If it is a rimless spectacle 
we first note the drilling to see that the holes are on a line and 
also that they are the correct distance from the edge. Should 
one or more of the holes be drilled a little above or below the 
Inspecting the Mountings 
Figure 204 
line the strap will necessarily be fitted at an angle. It also gives 
the lens the appearance of being out of shape. If it has been 
drilled "off" but slightly the strap can be removed and the hole 
filed a little up or down, as the case may be (Figure 204). This 
will naturally enlarge the hole to some extent, but the strap will 
remain fairly tight. 
This remedy is resorted to in cases of emergency only. The 
most satisfactory way is to fit a new lens, and this is usually done 
in the large shops. If it is found that the hole has been drilled 
too near the edge the strap can be bent down slightly with a pair 
of No. 35 strap pliers. The correct method is to make a slight 
kink near the post (Figure 205) and under no circumstances MECHANICAL OPTICS 
211 
should the points be bent down, leaving an opening between the 
lens and the strap (Figure 206). This operation is explained fully 
in Chapter VIII under the heading "Mounting." 
Figttre 205 
At this point should also be noticed whether the strap has 
been fitted properly to the surface of the lens, as illustrated in 
the same article. If the strap has been bent to a wedge shape it 
Figure 206 
will be possible to grasp each lens and twist the lens in the strap 
considerably. This not only throws the glasses out of alignment, 
but is very liable to cause the lenses to chip near the straps, as 212 
MECHANICAL OPTICS 
the lens-bearing parts slip back and forth on the edge. If the 
straps are not correctly fitted in this respect they can easily be 
removed and fitted with the strap pliers. It is unnecessary to 
fit new lenses as the holes are not changed materially in this 
operation. When inspecting the drilling it is unnecessary to 
have the spectacles in perfect alignment, although it is perhaps 
easier to detect any variation in drilling. With practice one soon 
learns to judge each lens separately and even though the mounting 
may be all out of shape, it is a simple matter to tell if the straps 
are fitted correctly. 
The next point is to inspect the frame or mounting and see 
if the material ordered has been used. Is it steel, gold or filled? 
In other words, is it what the prescription calls for? Mistakes 
are easily made in numbers, and you may find that gold-filled has 
been supplied when gold is called for, or vics versa. The differ- 
ence in the metal can be distinguished by the trade-marks, but a 
trained eye will readily detect the difference by the appearance 
and style of the mounting. The temples, if a spectacle, should 
be measured for length; also as to whether regular or cable 
temples are wanted. Mistakes of this character are very frequent, 
and with so much detail to remember, it is a simple point to over- 
look. 
If the prescription calls for eyeglasses it is more complicated, 
as the style and length of spring, the style of guards and the 
length of studs must be noted. With fingerpiece mountings this 
is simplified, as the style and number only require attention. 
The size being designated by the number, we soon learn to judge 
the correct mounting. 
Figure 207 
After inspecting the lenses and mountings and everything hav- 
ing been found to be correct, the glasses must now be put in con- 
dition to deliver or mail, as the case may be. The first operation 
will be to "line up'' the mounting; in other words, bring the 
straps into position, so that when the spectacle is held endwise 
the heads of the screws will form a straight line. Then hold the MECHANICAL OPTICS 
213 
spectacle so that you can look down on the edge and see if the 
lenses are straight in this respect (Figure 207). If they are not, 
they will have to be brought into line by bending one shank up 
or down, as the case may be. For this operation the shank is held 
with a pair of flat pliers, either snipe-nose, flat or parallel jaw. 
The No. 40 plier, which is used for adjusting fingerpiece mount- 
ings and for holding endpieces, on account of the narrow parallel 
jaw, is very convenient for this work also. The top of the bridge 
is grasped with the thumb and forefinger of the left hand and the 
Figure 208 
shank of the bridge with the pliers in the right hand (Figure 207). 
The bridge can then be thrown up or down with the left hand and 
the pliers in the right hand held stationary, or the bridge held 
securely with the left hand and the shank bent with the pliers in 
the right. If judgment is used before changing the bridge, it is a 
very easy matter to learn which way to throw it, but to the inex- 
perienced it may be found that although the spectacles have been 
straightened, the dimensions will not be correct. In this operation, 
it will be seen that we either make the bridge higher and with 
more inset, or lower and more outset. 
As we have just been working on the shanks of the bridge, it is 
just as well to inspect the height first. If it is too high or too low, 
bend it by the method just described, bending both shanks, of 
course, the same amount. Next turn the glasses endwise and 
inspect the inset or outset. If the mounting has been properly 
selected and bent to measurements by the bender, the raising and 
lowering of the bridge to correct height should also place the crest 
in the proper position as to the inset or outset. If these are not 
found to be correct it must be changed at this point. 214 
MECHANICAL OPTICS 
At this time it is well to measure the pupillary distance and 
also the base of bridge, as, if we find it necessary to change one 
measurement, a variation at some other point maybe of assistance 
in some way. For example, if the pupillary distance is too great 
Figure 209 
and the base of the bridge is also wide, it will, perhaps, be only 
necessary to bend the crest of the bridge, so as to make the base 
narrower, and this will also change the pupillary distance the 
required amount. If the base is too narrow the operation will be 
vice versa. By this it will be seen that in the inspection of a pre- 
Figure 210 
scription the bridge having been bent to the required dimensions 
before mounting, the work calls for slight changes only. Each 
variation is so slight that one helps the other, provided, however, 
that the bridge has been properly selected and bent correctly. 
When selecting a bridge for a prescription and bending it to MECHANICAL OPTICS 
215 
measurements, it requires more skill, whereas the slight changes 
that are necessary in inspecting can be accomplished easily by 
anyone having had a little practice. 
Figure 211 
The bridge having been straightened and bent to dimensions, 
we turn our attention to the endpieces and temples. First, see 
that the temples set back the right amount; also that they are 
even, and one not set back more than the other. The prescription 
will probably give the distance between the temples; this.distance 
is measured one inch from the endpieces. Some opticians make 
Figure 212 
the mistake of measuring a frame between the temples at the end- 
pieces, but this is not correct. The distance at the endpieces can 
be 4 inches and at a point 1 inch from the endpieces it can be 
inches (Figure 209). 
Temples can be set back almost any amount, except that in 
extreme cases, for very full faces, it is necessary to use extension 
temples, or long endpieces. There is a question, perhaps, which 
looks the best, but although the long endpieces (Figure 210) may 
look the best off the face, the extension temples (Figure 211) are 
very much neater on the face. The long endpieces extend out 216 
MECHANICAL OPTICS 
so far that they are very conspicuous, whereas the extension 
curving around the temples of the patient looks very neat. 
Extension temples can be obtained from the prescription 
houses, or they can be bent very easily with pliers. Always select 
a length one-half inch longer than required, as the bend will take 
up about that amount of stock. In setting back the temples the 
tips can be filed if steel or gold, but in gold-filled it is better to 
use a pair of No. 42 endpiece pliers to hold the joint (Figure 212). 
The joint is held by the plier in the right hand and the temple 
Figure 213 
grasped with the thumb and forefinger of the left, as close to 
the joint as possible. The temple is then bent backward the 
desired amount and, if held as described, the bend cannot be 
noticed. Do not bend them as illustrated in Figure 213, as they 
look badly. 
The temples should then be trued with thumb and forefinger so 
that there will be no kinks in them; also so that the straight part 
is perfectly straight and a natural curve to the tips. See that the 
temples curl the same way and as you hold the spectacle squarely 
in front of you with the temples curling toward you, each will be 
straight and not bent differently. If they are bent in this manner 
they do not fit the ears and also do not look well when the temples 
are closed. If the prescription calls for angular temples bend the 
endpieces, holding them with the No. 40 plier, and with a snipe- 
nose plier tilt the joint the required amount. The temples are 
also brought into alignment in the same manner (Figure 214). 
Do not bend the temples with hollow chop pliers so that they are 
all out of shape (Figure 215). It should be noticed if the temples 
fit well in the joints; if too loose they fall down, and this gives a 
very poor appearance to the job. ME CHA NIC A L OP TICS 
217 
If frameless this can be regulated by the screw in the end- 
piece, but in frames the endpieces must be brought together 
better, and this is accomplished by removing the screw and the 
lens if desired, but this is unnecessary, as it can be held in the 
frame with one hand. With a pair of snipe-nose pliers in the 
Figure 214 
right hand one endpiece is grasped and bent slightly towards 
the other. The screw is then replaced, and as the endpieces are 
brought together they bind on the outer end, so that the temple 
is held tightly. 
Figure 215 
When screwing the endpieces together be very careful that the 
dowel does not bind before it slips into the hole, as this will cause 
the endpieces to spread again. It is carelessness on the part of 
the grinder, when inserting the lenses in the frame, in allowing 
the dowel to strike against the opposite endpiece, thereby spread- 
ing them. This can be done either when pulling the endpieces 218 
MECHANICAL OPTICS 
together with pliers, or when inserting the screw. Gold-filled, 
especially, will not stand abuse in this manner. 
The inspection is now complete and the lenses should be 
cleaned. In many places the inspectors simply breathe on the 
lenses and wipe them with a rag. This is not only unsanitary, 
but it does not clean the lenses properly. An atomizer containing 
water or some antiseptic mixture is much better, as the spray 
cleans around the straps and also leaves a polish on the lenses. 
A rubber atomizer is very convenient, as it can lie in any position 
on the table. Glass towels are the best for wiping lenses and if 
a clean one is used each day there will be no danger of scratching. 
The axis can then be dotted if they are to be sent out in this way. CHAPTER XIII 
Stretching, Shaping and Repairing 
Zylonite and Shell Frames 
THE early impression that the wearing of Zylonite frames 
was a fad that would be of short duration has never been 
entirely eradicated. With the new styles and higher 
quality frames that are now being manufactured, there is no 
doubt but that this is a style that has come to stay. While these 
frames are made in round eyes, oval and drop-shaped lenses can 
be fitted as well, as the material can be softened by heating and 
stretching or shrunk as the case may require. 
Zylonite frames, like other frames, will break or wear out and 
the optician will be called upon to make numerous repairs, such 
as fitting temples, hinges or joints, rims or even new fronts. 
Material for this work can be obtained from any supply house, 
but as there are many different styles it would be well to have an 
assortment of joints and temples both bent and unbent. 
Temples can be finished after fitting by using a file to take off 
the rough places and finished or polished on the tripoli wheel and 
rouge buff. 
Figure 216 
On all new work care must be used in fitting the lenses into 
frame that you do not burn or twist the frame out of shape and 
this is another case where practice will be needed. 
219 220 
MECZL4JVZG4L OPTICS 
In shaping and stretching all Zylonite goods, it has been the 
almost universal practice, especially among the smaller shops 
and those opticians who do their own work of this nature, to use 
hot water to soften the material and make it pliable enough to 
work. 
And this method is strongly recommended by some manufac- 
turers of Zylonite goods. One of them advises the use of boiling 
hot water in which a small quantity of any heavy oil or salt is 
poured to prevent the Zylonite changing color. 
Figure 217 
Experience has proven that the use of dry heat obtained from 
an electric cementer's stove or similar heater provided with a top 
to retain the heat where it is needed, is very desirable for this 
work. The dry heat does not injure the Zylonite and is cleaner 
than the water method. 
The high polish on the all-Zylonite frames supplied at present 
should be preserved at all times and your customer is entitled to 
receive a frame with the finish just as it comes from the factory. 
It is, therefore, considered by many more satisfactory to use the 
electric heater or other dry heat in handling this class of goods. MECHANICAL OPTICS 
221 
An ordinary flat top electric stove (Figure 216) is frequently 
used for heating Zylonite frames for softening, but some heaters 
are especially designed for the purpose. 
A feature of one heater is a perforated top which allows hot air 
to come up and heat small portions of the frame. Temple and 
bridge adjustments are made by placing that part of frame to be 
reshaped directly in the hot blast. 
Figure 218 
The electric stretcher has a complete electric heating unit 
enclosed in a finely polished aluminum cone, securely mounted in 
an iron base (Figure 217). It is thoroughly insulated and comes 
ready for instant use with long cord and plug attached. The cone 
is accurately graduated from 36 mm. to 44 mm. 
For use where electricity is not obtainable there is an alumi- 
num cone (Figure 218) which can be used over a gas or alcohol 
lamp with equal satisfaction. 
When stretching the frame, after the water method has been 
used, it should be forced down evenly all around and, when the 
desired size is reached, dipped in cold water while on the block.