VISION. 281 



focusing power of such a lens will depend on two things : first, 

 its index of refraction, and, secondly, the curvature of its sur- 

 faces. 



A considerable part of the actual refraction of the rays 

 which enter the eye is accomplished at the curved surface of 

 the cornea, a smaller degree of refraction taking place at the 

 lens itself. The reason for this is that the refractive index 

 from air to cornea is much greater than that between the lens 

 and the humors of the eye in which the lens is suspended, these 

 humors and the cornea having very much the same refractive 

 indices. The entering rays are therefore refracted at two 

 places in the eye, namely, at the anterior surface of the cornea 

 and on passing through the lens. 



Fig. 52. Formation of image on retina. O.A. is the optic axis. 



Accommodation of the Eye for Near Vision. When the eye 

 is at rest, its optical system is of such a strength that parallel 

 rays, i. e., rays that are reflected from objects at a distance, are 

 brought to a focus exactly on the retina. The picture thus 

 formed is, however, upside down just for the same reason that 

 it is so on the screen of a camera (Fig. 52). When the object 

 looked at is so near that the rays reflected from it are divergent 

 when they enter the eye, it becomes necessary, if the image is 

 still to be focused on the retina, that some adjustment take place 

 in the optical system of the eye. This could happen in one or 

 two ways, either by the distance between the lens and the retina 

 becoming lengthened (the method used in a camera), or by in- 

 creasing the convexity of the lens. The former process cannot 



