NERVE 



145 



of lower to one of higher refractive index, so that the 

 peripheral rays are bent towards the perpendicular. Since 

 in the horse the curvature of the cornea is less in the 

 horizontal plane, rays of light in this plane are less refracted. 



2. At the anterior surface of the lens, from lower to 

 higher, so that the rays are again bent in the same way. 



3. At the posterior surface of the lens, from higher to 

 lower refractive index, so that the rays are bent away from 

 the perpendicular, that is, again towards the axial ray.V 



The degree of bending depends upon — 1st, The difference 

 of refractive index : 2nd, The obliquity with which the light 



Fig. 65. — 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. 



hits the surface. This will vary with the convexity of the 

 surface. 



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 proportionately to the curvature 

 of each. 



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

 pound lens composed of a convexo-concave part in front. 

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