VISUAL JUDGMENTS. 



181 



THE SIZE OF FAMILIAR OBJECTS is used to judge distances. If 

 the images on the retina of people or animals are small we assume 

 that they must be far away and other things which are evidently 

 near them, of which we do not so accurately know the size, are 

 judged also to be distant. 



BINOCULAR OR STEREOSCOPIC VISION. When we look with both 

 eyes at a flat object the image which is thrown on one retina is of 

 exactly the same shape as that made on the other. If the object 

 has depth, however, the two images are not exactly alike. The 

 right eye sees a little more of the right side and a little less of the 

 left than the left eye does, and vice versa. For instance, if one 

 looks from above at a pyramid with its top cut off, the right eye 

 receives an image of it of the shape of diagram A in Fig. 50,. 

 the left, one like B. Each of these diagrams alone looks perfectly 



FIG. 50. To illustrate binocular vision. 



flat. But hold them in front of your eyes about six inches away 

 and stare straight through them, relaxing your accommodation; 

 the two appear to fuse and the picture that results is one of a pyra- 

 mid which has depth as well as breadth and height. This is in all 

 probability the main factor in the better judgment of depth which 

 one gets from using both eyes. The principle is used in the stereo- 

 scope. Two photographs are taken of the same view, but the first 

 from a position a little to the right of that from which the second 

 is taken, the distance between the two positions being greater than 

 the distance between the eyes. These pictures are mounted on a 

 frame and, by an arrangement of prisms, the image of the one which 

 was taken from the right side is thrown on the right eye, and the 

 image of the left picture on the left. The result is a view in which 

 the relief or depth appears even greater than it really is. 



