PROCEEDINGS 



OF 



THE ROYAL SOCIETY. 



February 16, 1893. 



The LORD KELVIN, D.C.L., LL.D., President, in the Chair. 



A List of the Presents received was laid on the table, and thanks 

 ordered for them. 



The following Papers were read : — 



I " On a Portable Ophthalmometer." By Thomas Reid, M.D. 

 Communicated by Lord Kelvin, P.R.S. Received January 1, 

 1893. 



The object of the instrument about to be described is to measure 

 the curvature of the central area of the cornea, the polar or optical 

 zone ; or of any spherical reflecting surface of from 6 to 10 mm. of 

 radius. In its present form the instrument can only be applied to 

 the measurement of the corneal surface in the visual line. As this 

 is the area of the cornea utilised for distinct vision, this instrument 

 furnishes all the data practically requisite for the diagnosis and 

 measurement of corneal astigmatism. 



The theory of its construction is based on a particular application 

 of the following well-known optical law : that when two centred 

 optical systems are so combined that their principal foci coincide, 

 the ratio of the size of the object to the size of the image formed by 

 the combined systems is equal to the ratio of the principal foci of the 

 two optical systems, adjacent respectively to object and image. The 

 two optical systems in this case are the convex lens of the instrument 

 and the cornea as a reflecting surface, with the object in the prin- 

 cipal focus of the adjoining optical system. 



Thus : — (vide fig. 1). 



Let MM' be the convex lens of known focus, A the corneal surface, 

 and P' the point where their principal foci coincide. 



VOL. liii. b 



