NEURO-MUSCULAR MECHANISM 



123 



vitreous. The refractive indices of these, compared with air 

 as unity, may be expressed as follows : 



Cornea . 

 Aqueous 



1-33 

 1-33 



Lens 

 Vitreous 



1-45 

 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 cornea ; 



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, 



FIG. 54. To show how parallel rays are brought to a focus on the retina by 

 refraction at the three surfaces (a), anterior surface of the cornea ; (b), 

 anterior surface of the lens ; and (c), posterior surface of the lens. 



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 convexity, 

 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. 



These media, in fact, form the physiological lens, a com- 



