A NEW SERIES OF METRIC TEST-LETTERS 
FOR DETERMINING THE ACUITY OF 
DIRECT VISION FOR FORM. 
bv J 
CHARLES A. OLIVER, M.D. 
Philadelphia. 
A Paper read before the American Ophthalmological Society at the Annual 
Meeting held at the Pequot House, New London, Conn. 
July 15th and 16th, 1885. 
EXTRACTED FROM THE TRANSACTIONS OF THE SOCIETY. A NEW SERIES OF METRIC TEST-LETTERS FOR 
DETERMINING THE ACUITY OF DIRECT 
VISION FOR FORM. 
(Based upon Snellen and DeWecker.) 
By CHARLES A. OLIVER, M.D., 
PHILADELPHIA. 
The accompanying sheet of test-letters has several advan- 
tages not possessed by any other single form of test-card. 
These are as follows : 
First,- The Letter. In order to accurately fulfil the con- 
dition of Snellen's basis of letter formation, it was found that 
the nineteen letters, 
ABGHIJKMNPQRSUVW 
had to be expunged, leaving the letters 
C D E F L O T 
for employment. Each letter used embraces a square''area 
included in a five minute angle for both height and breadth, 
and each stroke of the letter is exactly one minute angle in 
width : further, there is a one minute angle or more of spacing 
everywhere throughout the breaks and openings in each let- 
ter. It will be thus seen that the entire letter is confined to 
a vertical and a horizontal angle of five minute opening, and 
that its component strokes are comprised in visual angles of 
one minute each, whilst in all situations throughout the square, 
the free spaces are not less than one minute angle in width. 2 
The regular block-letter has been used in preference to any 
other style of type, on account of its greater conformity to 
the average variety of character used in printing, which, as is 
well known, has a foot, head and tail piece ; care having been 
taken to have each piece occupy a square of one minute visual 
angle. 
r x o t a e c 
T 0 F C L D E 
E T C L D P O 
O r D E C T 
D E C F O 
E D T O L 
L C F D 
C E O T 
DEC 
E D T 
O F 
Second,- The Seqitence. Every line is so arranged that its 
letters follow each other in a certain definite order, though 
this is rendered incomprehensible t6 the patient by a system 
of irregular placing. Amongst the seven selected letters, 
there are four which are composed of vertical and horizontal 
strokes, and three which are made up of curved ones, thus 3 
giving a series of letters which contain radii at all possible 
angles. Nearly every line of letters has a complete combina- 
tion of such strokes situated upon it, so disposed as to make a 
series of vertical and horizontal strokes follow a series of 
curved ones ; by this means allowing each line to serve as an 
important adjuvant in the detection and the correction of 
astigmatism. 
Third,-The Interspace. Each letter is surrounded by a 
space which is greater, or at least equal to its own area. This 
has been accomplished first, by making each of the intervening 
spaces between the letters on the same line, of the same area 
as that occupied by the letter itself; and secondly, by sepa- 
rating the lines from each other by a space equal to the height 
of the lower letter. It might give the card a more aesthetic 
appearance to group the larger letters nearer together, but 
appearance has been sacrificed, and the letters kept at their 
proper distances, in order to preserve scientific unity through- 
out. 
Fourth,- The Gradation. In a desire to avoid unnecessary 
mental effort by the use of irregular fractions, and by a wish 
to render computation easy and quick, f instead of f has been 
taken as f. The order is as follows : 
2 I 2 I I I I I I I I I 
I, I, 3, 2, 3, 4, 5, 6, 7, 8, 9, 10. 
Gradation In 
Fractions. 
V V V VVVV V V V V V 
iiTr? v, viiTr? x, xv, xx, xxv, xxx, xxxv, xl, xlv, l. 
Gradation in 
Dioptrics. 
This mathematical scale was partially made use of by 
DeWecker, but, unfortunately, it never gained sufficient popu- 
larity to be universally employed. The advantages possessed 
by this change are almost incalculable, and can be justly com- 
pared with the superiority that the decimal system of coinage 
has over the various other methods of money naming and 
valuation. 
Fifth,- The Reverse Order. By this arrangement, two ob- 
jects of great consequence are gained. First, time is saved 
by the want of necessity to have the patient read a great many 
letters before the proper line is reached ; and secondly, a 4 
greater probability of correct answers by reason of the eye not 
having time to become fatigued. 
It may be of interest to explain the mode of manufacture of 
these types. The size of number L type was carefully 
estimated, and accurate India-ink sketches of the seven letters 
were drawn to that scale. Photographic reductions to the 
variously chosen sizes were obtained, the correctness of these 
being verified by exact measurement. Reverse phototypes 
were now made of each letter. These were tacked on a large 
sheet of white pasteboard in the order as they appear on the 
regular card. They were now rephotographed and rephoto- 
typed exactly to the same size.1 
For the conscientious and painstaking manner in which the 
card has been prepared, the writer desires to thank the 
Messrs. J. W. Queen & Co., of No. 924 Chestnut St., Phila- 
delphia, of whom copies may be obtained. 
1 Reading types based upon the same plan, comprising the same letters, arranged in 
a series of small words varying from 0.5 D. to 111-11 D,, are now in the process of 
construction, and will be published shortly.