VI FORM ANID FUNCTION * 123 
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 are 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 are sensitive cells, so that a mosaic picture is 
formed. There can be no doubt that eyes of this 
description are 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 witha 
biconvex lens in front—ze.,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—ze., 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 versd. Here, then, 
we have one means of focussing objects at different 
distances; we use lenses of various degrees of 
convexity. 
If the lens is to form a really clear image the light 
must fall upon it only from in front ; rays from the 
