430 Measurement of Astigmatism and other Ocular Defects. 



found to differ from that previously obtained, being usually 

 less. The difference between the two is the amount of astig- 

 matism of the eye. 



In what precedes I have supposed the principal meridians 

 of the eye to be horizontal and vertical. The modifications 

 required by their deviation from these directions are suffi- 

 ciently obvious. The amount of the deviation may be found 

 by means of a series of concentric circles with intervals of 

 about the same breadth, say *025 inch, view r ed directly. 

 When this object is placed on the table and moved up slowly, 

 a band of clearness at last appears along a nearly horizontal 

 diameter. This diameter shows one meridian, the other is 

 shown by the tangent at the middle of any of the clear seg- 

 ments, and these are the directions in which the lines above 

 called horizontal and vertical are supposed to lie. 



8. As an example illustrating the use of the apparatus, I 

 will take an actual case, remarking that each result given is 

 the mean of several observations. 



L, R, v, A, stand for left eye, right eye, vertical lines, hori- 

 zontal lines. 



Referring to the position, in the scale, of the punctum remotum, 

 I found Lv=-200, L/U-115, Rr=-216, RA=-144. 

 These observations show that the eyes are both greatly affected 

 by hypermetropia and astigmatism, for in a perfect eye all 

 these numbers are equal and do not differ much from *. 



LA — Li' = 85 is the astigmatism of the left eye; and 

 Wi-Rv = 72, that of the right. 



9. To remedy these defects I had glasses ground, of the same 

 form for both eyes. They were intended to have a spherical 

 convex front surface, of power f 212, and a concave cylindrical 

 back surface with axis horizontal, of power 62. 



The eyes while using these glasses were tested, and gave 

 the following results: — 



Lv=U, LA=37, Ru=15, RA = 14. 



These show that the glasses very nearly destroy the astig- 

 matism of the right eye and leave 23 of astigmatism in the 

 left, which was in several respects an inferior instrument. 

 Also by comparing these numbers with those given above we 

 may deduce the forms of the lenses. Thus : — Lv is increased 



* The meridians of greatest and least curvature were not exactly 

 horizontal and vertical, but the difference was small enough to be 

 neglected. 



t By this I mean the power of a lens having the surface named on one 

 side and a plane surface on the other. 



