264 



SCIENCE. 



[N. S. Vol. XIV. No. 346. 



considered the senses from the point of view of 

 psychology. The ordinary man tends to regard 

 our perceptions as copies or models of an out- 

 side world. The sense organs intervene, of 

 course, and knowing that in vision the rays of 

 light form an image on the retina, one is likely 

 to think of the mind as viewing this picture 

 from behind. As Professor Thorndike re- 

 marks, if one walks toward a chair, the appar- 

 ent size of the object does not alter — at least 

 not greatly or obviously — although the image 

 on the retina becomes smaller. As I lecture 

 to my class, the retinal images of the heads of 

 the students on the back seats may be only 

 one-tenth the size of those in front, yet they 

 look only slightly smaller. But this is by no 

 means the only disparity between the image on 

 the retina and my perception. To begin with, 

 the image is upside down and there are two 

 images. Then the corners of my desk look 

 like right angles, although they ' are by no 

 means such in the pictures, I know by experi- 

 ment that I can at one time have a distinct 

 image of only one of the nearer heads, yet in 

 my perception all the heads are distinct. A 

 photograph in which very near and compara- 

 tively distant objects are included will in its 

 distortion of perspective and blurring give 

 some idea of what the image on the retina is 

 like and how different it is from the perception. 



Some of these facts were known to Berkeley, 

 who in his ' New Theory of Vision,' first pub- 

 lished in 1709, argues that visible objects are a 

 system of arbitrary signs. In regard to ap- 

 parent magnitude he writes : 



When we look at an object, the tangible figure and 

 extension thereof are principally attended to ; whilst 

 there is small heed taken of the visible figure and 

 magnitude, which, though more immediately per- 

 ceived, do less sensibly affect us, and are not fitted to 

 produce any alteration in our bodies. 



60. That the matter of fact is true will be evident 

 to any one who considers that a man placed at ten 

 foot distance is thought as great as if he were placed 

 at a distance only of five foot ; which is true, not 

 with relation to the visible, but tangible greatness of 

 the object : the visible magnitude being far greater at 

 one station than it is at the other. 



Professor James, as always, discusses the 

 matter admirably. He says (' Principles of 

 Psychology,' 11. , 179): 



"When the object by moving changes its relations to 

 the eye the sensation 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 overlooking of retinal 

 magnitude become that it is next to impossible to 

 compare the visual magnitudes of objects at different 

 distances without making the experiment of super- 

 position. * * * As I look along the dining-table I 

 overlook the fact that the farther plates and glasses 

 feel so much smaller than my own, for I know that 

 they are all equal in size ; and the feeling of them, 

 which is a present sensation, is eclipsed in the glare 

 of the knowledge, which is a merely imagined one. 



Professor Newcomb recently called my at- 

 tention to the paradox that while we are 



Fig. 1. The man and boy are of the same size. 



said to judge the distance of objects of 

 known size by the angles they subtend, our 



