542 PHYSIOLOGY 



to the angle DCN' ( = a). Similarly the angle of refraction CBn is equal to the angle 

 BCn' ( = /3). Let h be the thickness of the glass plate. Produce DC backwards to 

 A'. It is required to find the perpendicular distance between A and A' ( = x). 



Now -^ = sin EGA' 



-DO 



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

 = sin (a ft). 



And = cos ft. 



Therefore x = h. 



sin (a - ft) 

 - -. 



cos ft 



If there are two such plates, arranged as in Fig. 269, then 



ab = 2x 



. 



cos ft 



The angle a is measured by the instrument ; the angle ft is calculated from the 

 formula for refraction, sin a = n. sin ft ; 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 altera- 

 tion 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 curvature 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 

 consistence 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 mem- 

 brane 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 leaves the sclerotic and hangs as a 

 curtain into the cavity of the aqueous humour, between the cornea and the 

 lens. The curtain, which is known as the iris, presents a circular orifice at 



