, PERCEPTION 297 



to different combinations of the stimulated substances. 

 Colour blindness on this view is due to the absence of one 

 or more of these substances, or to abnormality in their 

 absorption of light. 



Bering's Hypothesis. — There are three substances present 

 in the retina called red-green, yellow-blue and white- 

 black. These are capable of being cataboHsed or ana- 

 bohsed. When, for instance, the red-green substance is 

 stimulated by red rays it is built up into a more complex 

 compound, while under the influence of green rays it is 

 broken down. On this view colour bhndness to red and 

 green or to blue and yellow is due to the absence of the 

 corresponding substance. 



Edridge-Green's Hypothesis.— As in Young's view, three 

 substances are present, responding to red, green and blue, 

 but these are located in the brain. 



THE PERCEPTION OF SIZE, SHAPE AND DISTANCE 



When we look at an object with one eye we are dependent 

 for our idea of its size, shape and distance upon our past 

 experience. Into this several factors enter : (1) from our 

 knowledge of the true size of the object we can gauge its 

 distance ; (2) from the intensity of hght upon its diflerent 

 surfaces we can tell its shape ; (3) from the apparent con- 

 vergence of hues which we know by experience to be 

 parallel we can judge how far the lines recede ; (4) finally, 

 from parallex — that is, the relative movement of distant 

 and near objects as we move — we can estimate distance. 

 It is not possible that the muscular movements concerned 

 in accommodation give us a sensation of depth and 

 distance. 



With uniocular vision this power of judgment would 

 fail us if we were faced with conditions of which we had 

 no past experience. Even when we look at familiar 

 objects and scenes these always seem flatter to one eye 

 than to both eyes. 



