l8o UNIVERSITY OF COLORADO STUDIES 



its own elasticity, the two circles AB and CD will be concentric and in 

 the same plane, and we will have the two cones of rays for each eye, and 

 its two circles on the interposed plane, concentric, as is shown in Fig. 3. 

 If now the circle CD is pulled in toward the eyes the angle DXB becomes 

 smaller, and CXA larger, for the left eye, and vice versa for the right 

 eye; and if CD is pulled away from the eyes to the position of C"D", 

 directly the opposite will happen. So it can easily be seen why the 

 inside cones change their position to the right or left according as CD 

 is pushed in and out in the direction of depth. The two figures or 

 images (one for each eye) on the interposed plane of each of the above" 

 described figures constitute what we have called above the stereograph. 



This stereograph may be produced as follows: Place a piece of 

 celluloid or glass between the eyes and lamp-shade, as in Fig. 1. Now 

 with the head, interposed plane, and shade fixed and with the left eye 

 closed, trace as accurately as possible the image of the right eye on the 

 glass. Then, still in the same position, close the right eye and in a 

 similar manner trace the image of the left eye. By still keeping the 

 head and glass fixed and removing the shade, and by looking through 

 the glass in such a way that the traced image of each eye falls respec- 

 tively upon that eye it is impossible for you to tell that the globe has 

 been removed, for there it is in the same position and just as distinct 

 as when it was being drawn. 



How could the eye know that it is not the globe but the intercepted 

 images ? All the factors are the same. The eyes are converged 1 

 exactly to the same degree. The retinal images are exactly the same 

 size, of the same coloring, and in the same position, yet the object that 

 excites the retina is an entirely different thing and in a different position. 

 In fact each retina has its own image. Hence we have an illusion. 

 Give little Johnny the stereoscope and let him look at an interesting 

 picture — I mean scene. If you ask him to show you where a certain 

 object in the background of the scene is he will not put his finger on the 

 stereograph, but will stretch his arm away beyond it in the direction in 

 which it seems to him to be. 



1 By convergence is meant the bending in of the two eyes (Fig. 5). The farther an object looked at is 

 away the less the convergence, i. e., the more nearly parallel the eyes are. 



