THE EYE AS AN OPTICAL INSTRUMENT 259 



tance between the lenses and the ground glass, in common lan- 

 guage " focussing the instrument," either can be made distinct. 

 For near objects the lenses must be farther from the surface on 

 which the image is to be received, and for distant nearer. 



The Refracting Media of the Eye Form a Convergent Optical 

 System, made up of cornea, aqueous humor, lens, and vitreous 

 humor. These four media are reduced to three practically, by 

 the fact that the indices of refraction of the cornea and aqueous 

 humor are the same, so that they act together as one converging 

 lens. The surfaces at which refraction occurs are: (1) that be- 

 tween the air and the cornea; (2) that between the aqueous humor 

 and the front of the lens; (3) that between the vitreous humor 

 and the back of the lens. The refractive indices of those media 

 are: the air, 1; the aqueous humor, 1.3379; the lens (average), 

 1.4545; the vitreous humor, 1.3379. From the laws of the re- 

 fraction of light it therefore follows that (Fig. 87) the rays Cd 

 will at the corneal surface be refracted towards the normals N, N, 

 and take the course de. At the front of the lens they will again 

 be refracted towards the normals to that surface and take the 

 course ef; at the back of the lens, passing from a more refracting 

 to a less refracting medium, they will be bent from the normals 

 TV" and take the course fg. If the retina be there, these parallel 

 rays will therefore be brought to a focus on it. In the resting 

 condition of the natural eye this is what happens to parallel rays 

 entering it: and, since distant objects send into the eye rays which 

 are practically parallel, such objects are seen distinctly without 

 any effort, because all rays emanating from a point of the object 

 meet again in one point on the retina. 



Wide Range of Clear Vision in the Resting Eye. While in the 

 normal resting eye only parallel rays focus exactly on the retina, 

 the fact is that it sees clearly all objects that are as far as 18-20 feet 

 away. The rays of light from points on such objects are divergent 

 when they strike the cornea, and their focus is therefore behind the 

 retina. How then can the resting eye see such objects clearly? 

 The explanation is found in the structure of the retina. The light 

 perceiving elements, the rods and cones (Fig. 83), although ex- 

 tremely minute, are not mathematical points, but objects with 

 measurable diameter. When light falls on one of them the effect 

 is the same whether only a part of the element is illuminated or the 



