vi FORM AND FUNCTION 



^3 



to distinguish sight from mere sensitiveness to light: 

 Thus much even an earthworm possesses, for when at 

 night the light of a lantern is thrown upon him he 

 hurries into his hole. This is quite different from 

 seeing a definite image of things. With our eyes 

 shut, we can tell whether we arc in a bright light or 

 in the dark, and the earthworm has no power beyond 

 this. An insect's compound eye, again, is formed on 

 quite a different principle from the eyes of vertebrate 

 animals. It has a number of tiny facets beneath 

 which arc sensitive cells, so that a mosaic picture is 

 formed. There can be no doubt that eyes of this 

 description arc very inferior to our own. Among 

 their great defects is this, that they have no power 

 of adjusting themselves to different distances. To 

 return to the vertebrate eye. It is a camera with a 

 biconvex lens in front — i.e., a lens rounded outwards on 

 both sides. If a lens of this kind (a common magni- 

 fying glass will do well) be taken and a candle be 

 held in front of it, an inverted image of the flame 

 will be thrown upon the wall. The room must be 

 darkened except for the candle, and you must be 

 careful to get the right focus — i.e., to hold the lens at 

 such a distance from the wall, and the candle at such 

 a distance from the lens, that the image is clear. 

 The less convex the lens, the further away you 

 must hold the candle, and vice versa. Here, then 

 we have one means of focussing objects at different 

 distances ; we use lenses of various degrees of 

 convex it) T . 



If the lens is to form a really clear image the light 

 must fall upon it only from in front ; rays from the 



