DIOPTRIC MECHANISMS OF THE EYEBALL 603 



X 



EG 



Now - sin EGA 



= sin (n'CA' - n'GB) 

 = sin (a /3). 



And _=cos/3, 



, sin (a - /3) 



Therefore x = h. - i - -L-4 



cos /) 



If there are two such plates, arranged as in Fig. 267, then 

 a'b" - 2x 



cos p 



The angle a is measured by the instrument ; the angle (3 is calculated from 

 the formula for refraction, sin a = n. sin /3 ; and the thickness of the plates, h, 

 is known. Therefore the distance between the images can be calculated." 



In the normal eye in a position of repose, i.e. focused for parallel 

 rays, the curvatures of the three principal refracting surfaces are as 

 follows : 



mm. 



Anterior surface of cornea .... 8 

 Anterior surface of lens . . . . 10 



Posterior surface of lens .... 6 



During maximum accommodation the radius of curvature of the 

 anterior surface of .the lens changes to 6 mm. At the same time there 

 is a slightly increased curvature at the periphery of the posterior 

 surface, but the effects of this change are negligible as compared with 

 those produced by the alteration of the anterior surface. 



In order to determine the method in which this change in curvature 

 of the anterior surface of the lens is brought about we must refer in 

 some detail to the structure of the anterior half of the eye and the 

 manner in which the lens is hung up between the aqueous and vitreous 

 humours. The outermost layer of the eye is formed by the sclerotic 

 coat, a strong tough membrane of white fibrous tissue. In front this 

 is continuous with the cornea, which , having a smaller radius of curva- 

 ture than the globe of the eye, protrudes like a watch-glass from its 

 anterior surface. The main substance of the cornea is formed, like 

 the sclerotic, by white fibrous tissue, modified however in its consis- 

 tence so as to be perfectly transparent instead of white and opaque 

 like the rest of the sclerotic. Internal to the sclerotic is the choroid 

 coat, a membrane with a double pigmented internal layer, and supplied 

 with blood-vessels which furnish the vascular supply to the whole eye. 

 In front the choroid coat presents a series of folds, arranged in a 

 circle around the anterior part of the cavity of the vitreous and known 

 as the ciliary processes. In front of the ciliary processes the choroid 



