DESCRIPTION 
OF A 
REVOLVING ASTIGMATIC DISK. 
BY 
CHARLES A. OLIVER, M.D., 
ONB OF THE OPHTHALMIC AND AURAL SURGEONS TO ST MARY’S 
HOSPITAL, PHILADELPHIA. 
FROM 
THE MEDICAL NEWS, 
October 6, 1883. DORNAN, PRINTER. DESCRIPTION OF A REVOLVING 
ASTIGMATIC DISK. 
This instrument consists of three distinct parts: 
A flat brass rod, thirteen millimetres wide and fifty 
centimetres long, perforated at each end by a small 
hole. The rod is immovably fastened at its centre 
to a circular disk thirty-three centimetres in diam- 
eter, bearing on its face three concentric series 
of short test-lines, similar to those of Becker, each 
being representatives of a certain distance, width, 
and angle. Upon the periphery of this card are 
numbers representing five degree differences of 
angle, commencing at o° on the left-hand side of 
the horizontal meridian, running on the under 
half up to i8o° at the right-hand side of the same 
meridian. This is repeated in the opposite direc- 
tion upon the upper semicircle, thus making the 
angles correspond, similar to the ordinary test- 
glass frame. Between the front card and the re- 
taining rod there is a rotating disk forty-seven cen- 4 
timetres in diameter, having two of Pray’s letters 
placed at a quarter angle to each other (ninety 
degrees apart), each having a small arrow so fixed 
as to meet the number of degrees on the outer card, 
showing the exact angle reached. 
The primary principles in this instrument are in 
no way new, and for them no originality is claimed, 
as this as well as all other forms and contrivances 
are mere modifications of Thomas Young’s original 
optometer. The placing of two lines of equal 5 
width and blackness at right angles to each other, 
which, to be seen with equal distinctness, must, 
be set at different distances in reference to each 
other, if there be any asymmetry in the dioptric 
system of the eye. 
The disk is to be fastened to a wall by the re- 
taining rod, in a good light coming over the head 
of the observer placed at six metres’ distance, and 
at a height so arranged as to bring the centre of 
the card on a level with the patient’s eye. 
If we find him able to see the 6 D. type with the 
eye under examination, he is to have his attention 
confined to the lines of the inner circle. If the 
sight of the eye be defective, he is to look at the 
outer circles. By now making him closely watch, 
we ask him to designate the clearest, sharpest, and 
blackest radius; after this has been chosen, we 
wheel the striped letter Z to the angle named, or to 
a point between, and ask which letter is the darker 
and clearer. He will then answer that the Z ap- 
pears the plainer; now wheel the A into the place 
occupied by Z, and if it becomes the darker and the 
clearer, he will have verified his assertion, and the 
angles of astigmatism be obtained.1 
1 If the patient’s sight is so bad that he cannot make out any 
of the lines at six metres’ distance, he must gradually approach 
the test, and watch for the first brought out. We note the 6 
Its employment in the estimation of ametropia, 
whilst the eye is under the influence of a mydriatic, 
is also of much value. I have several times, with- 
out the use of any test-types whatever, made ex- 
perimental determination, and found that the re- 
sults were identical with those obtained during the 
use of the letters. Yet it is no more than fair to 
state, that it was done with subjects of more than 
average intelligence, as really, although forming a 
most delicate test, it is harder to appreciate and 
keep in mind the slight observable changes of dif- 
ference in the distinctness of the lines. 
As an adjuvant in the verification of a corrected 
case of ametropia, it is of incalculable advantage. 
I always endeavor, after the selection of the cor- 
recting lenses, to have the two letters appear equally 
black and distinct in their entire revolution of the 
primary card, and that every spoke in the inner 
wheel (or those of the concentric widths, if I 
cannot bring vision up to normal) shall be of 
the same clearness, by these means getting the 
amount and angle of astigmatism almost to a dead 
certainty. 
distance and angle, and place the letter Z at the chosen meridian. 
He is then to walk nearer and nearer until the letter A and the 
lines at its meridian become as distinct as the letter Z and its 
lines. We accurately register this new distance, and calculate 
the difference. 7 
These disks can be obtained from Mr. John L. 
Borsch, optician, No. 221 South Ninth Street, 
Philadelphia. 
1507 Locust Street, Philadelphia