J.— PSYCHOLOGY 205 



IV. Individual Differences in Visual Perception. 



Let us now return to the experiment with the inclined eUipse to note a 

 particular feature in it which is, I think, a characteristic of the perceptual 

 processes that has often been ignored. This feature is the wide range of 

 individual differences. Apart from such obvious differences as errors of 

 refraction, colour-blindness, etc., the optical system of different individuals' 

 eyes and consequently the conditions of local physiological stimulation 

 on the retina for a given arrangement of external objects is very much 

 the same. The perceptual responses of different individuals are, how- 

 ever, widely different, so that any two of us in the same physical sur- 

 roundings may create from them a very different phenomenal world. 



If two or three people perform the experiment I have just described, 

 we shall find that the height at which they say the apparent shape of the 

 inclined ellipse is circular is different to an almost incredible extent. 

 One may see the ellipse as circular when his head is only a few inches from 

 the table so that his retinal image is of a very much flattened ellipse, 

 while another sees the ellipse as circular when he is looking well down 

 on it so that his retinal image is itself not very far from circularity. The 

 first individual shows a very great effect of the real shape of the ellipse in 

 determining its apparent shape, the second shows a relatively smaller 

 effect of the real shape on apparent shape. 



It is true that the exact height at which each subject reports the ellipse 

 as looking circular is somewhat variable and may depend to some extent 

 on his mental attitude, but the limits within which variation occurs in 

 any one individual are small compared with the differences between 

 different individuals. If a subject showing little effect of the real shape 

 on apparent shape look at the ellipse from the height at which one with 

 small effect sees it as circular, he will report that by no effort of imagina- 

 tion can he make the ellipse look circular at that inclination, and he will 

 generally add that he does not believe that any one else can. 



That these are real individual differences and not merely accidental 

 variations in measurement is shown by the fact that they show great 

 consistency from one time to another. I once retested, after an interval 

 of two years, a group of twenty-five subjects for each of whom I had 

 measured the apparent shape of an inclined object. They differed widely 

 amongst themselves at each test, but the agreement between the two sets 

 of tests was extraordinarily high. The coefficient of correlation was 0-92, 

 which is as high as one expects to get in psychological measurements. 



There are, then, genuine and large individual differences between 

 different persons in the apparent shapes of inclined objects. We may 

 add that there are similar individual differences in the apparent sizes of 

 objects at different distances and in the apparent whiteness of objects 

 under different illuminations. In both of these cases, the same general 

 law holds. If an object is moved to twice its previous distance from our 

 eyes, it does not look half its previous size. It may, for different indi- 

 viduals, look three-quarters of its previous size or nineteen- twentieths. 

 With rare exceptions (which I shall mention later) the law holds that the 

 apparent size is in between the retinal size and the real size. In the same 

 way, if a piece of white paper is put into shadow so that it reflects less 



