152 UNIVERSITY OF COLORADO STUDIES 



In Fig. i, if we take the corresponding points a on the circles, left- 

 hand a stimulates the left eye, and the right-hand one the right eye. 

 The left eye refers the point of stimulus along the line XaA, and the 

 right eye along the line ZaA. But where those two corresponding 

 lines of reference meet there the point of stimulus seems to be. That 

 point is A. (The dash lines in these figures represent lines of reference.) 

 The supposed point of stimulus C lies in the two lines of reference. It 

 is in a plane closer to the eye than A, or any point in the circle AB, 

 consequently the eye interprets it as an extended object with three 

 dimensional values. In Fig. 3, all the points of reference AB-CD 

 are in the same plane, consequently the eye interprets the object as having 

 only two dimensional values. 



The most conclusive argument in favor of the importance of con- 

 vergence is found in a study of Figs. 13 and 14. Taking first Fig. 13, 

 where the little circles of the figures on the glass plate are eccentric 

 toward the inside, if the image of the right-hand figure falls on the right 

 eye and that of the left side on the left eye, what seems to be seen is the 

 large cone AcB. This will seem far away. If now the eyes are squinted 

 so that the left figure falls on the right eye and the right figure on the 

 left eye, what will be seen is A"C"B" , which seems small and close at 

 hand. The apex will also be turned away, whereas in the first case 

 (when it seemed far away) the apex was toward you. How can all this 

 be explained ? The size of the retinal images in both cases is the same. 

 The squinting has reversed the figures and this accounts for the change 

 of direction of the apex. In the case of squinting the convergence is 

 very much greater, and consequently the points of reference are brought 

 closer. Fig. 14 is just the opposite from 13, i. e., the images on the 

 glass serving as a stereograph are reversed, and the results can be 

 understood by consulting the diagrams. 



The question might be asked: How is it that I can compare the 

 statures of people at a distance with those immediately before me? 

 The retinal image of those close by may be many times as large as of 

 those farther off, and yet I can make as accurate comparisons between 

 close and far-off people as between those of equal distances. If you 

 were to represent by diagram the ocular relations for the two situations, 



