THE PERCEPTION OF SIGHT. 247 



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 estimat- 

 ing 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 inquiry 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 standing at different distances from the spectator. It is part 

 of a painter's merit for his figures to stand out boldly. Now 

 how does he produce the illusion 1 We shall find, in the first 

 place, that in painting a landscape he likes to have the sun 

 near the horizon, which gives him strong shadows; for these 

 throw objects in the foreground into bold relief. Next he 

 prefers an atmosphere which is not quite clear, because slight 

 obscurity makes the distance appear far off. Then he is fond 

 of bringing in figures of men and cattle, because, by help of 

 these objects of known size, we can easily measure the size and 

 distance of other parts of the scene. Lastly, houses and other 

 regular productions of art are also useful for giving a clue to 

 the meaning of the picture, since they enable us easily to recog- 

 nise the position of horizontal surfaces. The representation of 

 solid forms by drawings in correct perspective is most successful 

 in the case of objects of regular and symmetrical shape, such as 

 buildings, machines, and implements of various kinds. For we 

 know that all of these are chiefly bounded either by planes 

 which meet at a right angle or by spherical and cylindrical 



