THE PERCEPTION OF SPACE. 179 



keep their sizes, and that most of our sensations are 

 aflected by errors for which a constant allowance must be 

 made. 



In the retina there is no reason to suppose that the 

 bignesses of two impressions (lines or blotches) falling on 

 diflerent regions are primitively felt to stand in any exact 

 mutual ratio. It is only when the impressions come from 

 the same object that we judge their sizes to be the same. 

 And this, too, only when the relation of the object to the 

 eye is believed to be on the whole unchanged. When the 

 object by moving changes its relations to the eye the sensa- 

 tion excited by its image even on the same retinal region 

 becomes so fluctuating that we end by ascribing no absolute 

 import whatever to the retinal space-feeling which at any 

 moment we may receive. So complete does this overlook- 

 ing of retinal magnitude become that it is next to impossi- 

 ble to compare the visual magnitudes of objects at different 

 distances without making the experiment of superposition. 

 We cannot say beforehand how much of a distant house or 

 tree our finger will cover. The various answers to the 

 familiar question. How large is the moon ? — answers which 

 vary from a cartwheel to a wafer — illustrate this most 

 strikingl}". The hardest part of the training of a young 

 draughtsman is his learning to feel directly the retinal (i.e. 

 primitively sensible) magnitudes which the different objects 

 in the field of view subtend. To do this he must recover 

 what Ruskin calls the ' innocence of the eye ' — that is, a 

 sort of childish perception of stains of color merely as 

 such, without consciousness of what they mean. 



With the rest of us this innocence is lost. Out of all the 

 visual magnitudes of each knoivn object we have selected one as 

 the REAL one to think of, and degraded all the others to serve as 

 its signs. This ' real ' magnitude is determined by aesthetic 

 and practical interests. It is that which we get when the 

 object is at the distance most propitious for exact visual 

 discrimination of its details. This is the distance at which 

 we hold anything we are examining. Farther than this we 

 see it too small, nearer too large. And the larger and the 

 smaller feeling vanish in the act of suggesting this one, 

 their more important meaning. As I look along the dining- 



