THE STEREOSCOPE AND VISUAL INTERPRETATION 1 83 



it could be seen, that while, for the far situation, both the retinal image 

 and amount of convergence are small, for the nearer object both would 

 be much greater. So by a process of correlation which becomes estab- 

 lished in experience we seldom go wrong in our judgment. 



In Fig. 20, by combining 1 the upper two figures the little end comes 

 toward you. The middle one (2) remains flat, and the lower one (3) 

 projects away from you. Now squint. This can be done by holding 

 up the finger between yourself and the card. Look at the finger and 

 let the lines of sight cross at it. Now / will be away from you and 3 

 toward you, and all three figures will be small because of the increased 

 convergence. 



If you superpose the two images in Fig. 15, you will get a beautiful 

 third dimensional effect of bridgework. Fig. 17 is the two images of 

 15 reversed. So here the squinting is done for you. Now by super- 

 posing the two images, you get an extended effect of iron framework, 

 but with the little end toward you ; whereas, in 1 5 all the bends are 

 of the same size, and the sides parallel. Now squint for each of these 

 two figures. The figure which you get from 15 when squinting is the 

 same in design as 17 when not squinting; and 17 when squinting is 

 the same in design as 15 when not squinting. But in each case when 

 squinting the figure is smaller than when not squinting, and this is 

 accounted for as above by increased convergence. 



Figs. 5 to 8 represent stereoscopes constructed on the reflecting prin- 

 ciple. In Fig. 5, A" and B" represent mirrors with the reflecting faces 

 turned toward the eyes at an angle of about 45 . (The faces of all 

 these mirrors are represented by a straight black line and the backs by 

 a lighter corrugated edge.) A and B represent the objects looked at. 

 A represents the right half of the stereograph and B the left. These 

 are superposed by means of the two inclined mirrors. Notice here 

 that A and B are the objects from which the stimulus comes to the 

 eyes; but the point from which it comes to each eye is on the mirrors at 

 A" and B" . The eyes do not interpret them as coming from these 

 sources, but refer them on along the lines A" A' and B"B', and where 

 those two lines meet there the images are superposed and give third 



1 This can be done with very little practice. Hold up the card and place a finger beyond. Look at the 

 finger. Now move the card back and forth until the images come together and fuse. 



