A NEW OPHTHALMOSCOPE. 8. { 



Its mean distance is considered to be near the centre of the crystal- 

 line lens. 



For the sake of simplicity, in the accompanying diagrams, I have 

 represented the eye as a homogenous body, possessed of a single con- 

 densing, refracting surface, which may be regarded as the optical 

 equivalent of the various surfaces in a real eye, and may be considered 

 sufficiently accurate for any optical conclusions involved in the present 

 paper.* 



It is well known that under ordinary circumstances the pupil of the 

 eye appears to be perfectly black, and that all parts behind it are per- 

 fectly invisible ; this was formerly thought to depend on the complete 

 absorption of all the rays of light that fall upon the fundus or posterior 

 internal surface of the eye, so that none of them passed out aga,in 

 from its interior. 



That this is not the case can very easily be demonstrated by a 

 simple experiment suggested by Wharton Jones : — " Having previously 

 dilated the pupil of a cat's eye by a solution of Atropine or Bella- 

 donna, drop some water into the eye while the eyelids are held apart, 

 and cover the cornea with a thin plate of glass. The optic nerve en- 

 trance and the vessels of the retina can then be distinctly seen slightly 

 magnified." 



In this experiment we in reality neutralize the refracting condensing 

 power of the convex surface of the cornea. Here it will be seen that 

 the water, filling up the space between the cornea and the piece of 

 glass, forms a perfect concave lens with its concavity applied to the 

 cornea, thus changing the convex to a plane surface. From this it is 

 evident that as the fundus of the eye comes in view, when its refractive 

 power is to a certain extent neutralized, therefore the blackness of the 

 pupil and the invisibility of the parts behind it depend solely upon 

 the refraction of the light by the ocular media. 



This phenomenon of refraction may be demonstrated with any small 

 camera obscura by simply placing a piece of pasteboard behind the 

 ground glass so as to exclude all light from the camera except what 

 reaches it through the lens ; the ground glass* being in focus, distinct 

 images of objects in front of the lens are formed on its surface, 

 notwithstanding which, the interior of the camera when viewed 

 through the lens appears absolutely black. 



• The same mode of representation has been adopted by Stellway von Carion, Vienna, and 

 by G. Eainy, MD., Glasgow. 



