THE SENSES 105 



The idea of size depends upon the extent of the eye-cells 

 stimulated by the light coming from a body. If a large 

 surface is acted upon, the body seems large ; if a small 

 surface, the body seems small. But the extent of eye-cells 

 acted on depends not merely upon the size of the object, 

 but also upon its distance from the eye. Hence, our ideas of 

 size are judgments based upon the size of the picture in the 

 eye, and the appreciation of the distance of the object. The 

 distance of an object, when over fifty or sixty metres from the 

 eye, and very probably even when over as little as six metres, 

 is judged by the modifications in its shading and colour due 

 to the condition of the atmosphere. A range of hills will at 

 one time be judged to be quite near, at another time to 

 be distant. Since the estimation of the size of an object 

 depends upon the judgment of its distance, the estimation 

 we make of the size of such objects as a range of hills is often 

 most erroneous. When objects are nearer to the eye, a 

 special mechanism comes into play to enable us to determine 

 their distance (see p. 114). 



The idea of thickness or contour of an object is also largely 

 a judgment based upon colour and shading. When a cube is 

 looked at, we judge that it is a cube because of the degrees of 

 illumination of the different sides degrees of illumination 

 which may be reproduced in a flat picture of such a cube. 

 When the object is near the eyes, by using the two eyes 

 together a means of determining solidity comes into action 

 (see p. 124). 



When the manner in which we gain knowledge of our 

 surroundings by vision is analysed, it must be admitted that 

 the dictum " seeing is believing" has at best an unsubstantial 

 physiological basis, and that most of the points about any- 

 thing which we say we see e.g. its size, distance, and contour 

 are largely judgments formed by us upon a flat picture pro- 

 duced in the cells of the eye, which flat picture has in turn 

 led to these changes in our brain which are accompanied by 

 the changes in our consciousness upon which our judgment 

 has to act. 



Any defect in the visual mechanism must, and does, lead 

 to defects in the mental picture formed, and the accuracy of 



