THE PERCEPTION OF SIGHT. 281 



der, for we have already learned enough to see that the 

 questions which have here to be decided are of funda- 

 mental importance, not only for the physiology of sight, 

 but for a correct understanding of the true nature and 

 limits of human knowledge generally. 



Each of our eyes projects a plane image upon its own 

 retina. However we may suppose the conducting nerves 

 to be arranged, the two retinal images when united in 

 the brain can only reappear as a plane image. But 

 instead of the two plane retinal images, we find that 

 the actual impression on our mind is a solid image of 

 three dimensions. Here, again, as in the system of 

 colours, the outer world is richer than our sensation by 

 one dimension ; but in this case the conception formed 

 by the mind completely represents the reality of the 

 outer world. It is important to remember that this 

 perception of depth is fully as vivid, direct, and exact 

 as that of the plane dimensions of the field of vision. 

 If a man takes a leap from one rock to another, his life 

 depends just as much upon his rightly estimating the 

 distance of the rock on which he is to alight, as upon 

 his not misjudging its position, right or left; and, as 

 a matter of experience, we find that we can do the one 

 just as quickly and as surely as the other. 



In what way can this appreciation of what we call 

 depth, solidity, and direct distance come about ? Let 

 us first ascertain what are the facts. 



At the outset of the enquiry we must bear in mind 

 that the perception of the solid form of objects and 

 of their relative distance from us is not quite absent, 

 even when we look at them with only one eye and 

 without changing our position. Now the means which 

 we possess in this case are just the same as those which 

 the painter can employ in order to give the figures on 

 his canvas the appearance of being solid objects, and of 



