CHAPTER VI. 



THE SENSE OF SIGHT. 



It might at first sight seem easy enough to answer the 

 question whether an animal can see or not. In reality, 

 however, the problem is by no means so simple. We 

 find, in fact, every gradation from the mere power of 

 distinguishing a difference between light and darkness 

 up to the perception of form and colour which we 

 ourselves enjoy. 



The uudiiferentiated tissues of the lower animals, 

 and even of plants, are, as we all know, affected in a 

 marked manner by the action of light. 



But to see, in the sense of perceiving the forms of 

 objects, an animal must possess some apparatus by 

 means of which — firstly, the light coming from different 

 points, a, h, c, d, e, etc., is caused to act on separate 

 parts of the retina in the same relative positions ; and 

 secondly, by means of which these points of the retina 

 can be protected from the light coming in other 

 directions. 



There are three modes in which it is theoretically 

 possible that this might be effected. 



Firstly, let S S' be an opaque screen, with a small 

 orifice at o. Let a h e d e he a body in front of the 



