i io HUMAN PHYSIOLOGY 



with a small hole in the centre through which the observer 

 can study the illuminated part of the chamber. (Experi- 

 ment.) 



Physiology. The eye may be compared to a photographic 

 camera, having in front a lens, or lenses, to focus the light 

 upon the sensitive screen behind (Fig. 46). The picture 

 is formed on the screen by the luminous rays from each 

 point outside being concentrated to a point upon the screen. 

 This is brought about by refraction of light as it passes 

 through the various media of the eye the cornea, aqueous, 

 crystalline lens, and vitreous. The refractive indices of 

 these, compared with air as unity, may be expressed as 

 follows : 



Cornea . . 1'33 Lens . . 1'4~> 

 Aqueous . 1'33 Vitreous . 1'33 



Thus light passes from a medium of one refractive index 

 into a medium of another refractive index 



1. At the anterior surface of the corn* -a : 



2. At the anterior surface of the lens ; 



3. At the posterior surface of the lens ; 



and at these surfaces it is bent. The degree of bending 

 depends upon 1st, The difference of refractive index. 2nd, 

 The obliquity with which the light hits the surface. This 

 will vary with the convexity of the lens being greater the 

 greater the convexity. 



The posterior surface of the lens has the greatest con- 

 vexity, with a radius of 6 mm. The anterior surface of 

 the cornea has the next greatest, with a radius of 8 mm. 

 The anterior surface of the lens has the least, with a radius 

 of 10 mm. A ray of light passing obliquely through these 

 media will be bent at the three surfaces. 



x These media in fact form a compound lens composed of 

 a convexo-concave part in front, the cornea and aqueous, 

 and a biconvex part behind, the crystalline lens. In the rest- 

 ing normal eye (the emmetropic eye) the principal focus is 



