104 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



ject. Moreover, the image must be an exact facsimile of 

 the object, because what we see will be a facsimile of the 

 actual image. See, then, the two very distinct parts of 

 the eye, viz., the specialized nerve (retina) and the optic 

 instrument for making an image. The one is necessary 

 for the perception of light, the other for the perception 

 of objects. 



In order to bring out more clearly the distinctness 

 of these two parts I will use an illustration. Suppose 

 we eviscerate the eyeball — i. e., remove all its contents — 

 leaving only a deep cup-shaped cavity lined with the 

 retina, as can be, and indeed has been, done. Suppose, 

 further, that the retina could retain its healthy condition, 

 which, of course, is impossible but supposable. Then 

 I would undertake to have made a glass eye which, fitted 

 into the cup-shaped cavity, would see just as well as the 

 natural eye, and perhaps even a little better. 



The Necessity of Lenses. — The image must be a 

 perfect facsimile of the object, because what we see will 

 be a perfect facsimile of the retinal image. If there is 

 no image we will see no object. If the image is blurred 

 the object will seem blurred. If the image is clear and 

 sharp the object will be seen sharp in outline and clear 

 in all surface details. Now light passing through a 

 small hole will make an image (pinhole image), and 

 therefore a very small pupil would make an image on 

 the retina. But such an image is very imperfect. To 

 make a perfect image we must have a lens. The man- 

 ner in which a lens acts in producing an image is shown 

 in Fig. 73. It is seen that all the rays coming from 

 a point (a) of the object are bent (refracted) in such wise 

 as to meet one another at a point (a') of the image, all 

 the rays from the point b are gathered to one point b' 

 and all from c to c\ and similarly for all other points of 

 the object. Thus for every radiant point of the object 



