BINOCULAR VISION. 349 



non-corresponding parts of the two retinas. Four pictures, there- 

 fore will be seen, all out of focus. With a little practice one can so 

 converge his eyes as to make the two middle images come together, 

 and since one of these is an image of the right-eyed picture and is 

 falling on the right eye, and the other is a left-eyed picture falling 

 on the left eye, the combination of the two fulfills the necessary 

 conditions for binocular perspective. The figure stands out in 

 bold relief. 



Explanation of Binocular Perspective. Our perception of 

 solidity or relief is a secondary psychical act, and, so far as the binoc- 

 ular element is concerned, it is based upon the fact that the images 

 are slightly different on the two retinas; but why this dissimilarity 

 should produce an inference of this kind is not entirely understood. 

 Certain facts have been pointed out as having a probable bearing 

 upon the mental process. In the first place, in stereoscopic pictures, 

 as in nature, we do not see the whole field at once. To see the ob- 

 jects in the foreground the eyeballs must be converged by the eye 

 muscles so that the lines of sight may meet in the object regarded. 

 When attention is paid to objects in the background less convergence 

 is necessary (see Fig. 149). The point of fixation for the lines of 

 sight is kept continually moving to and fro, and the sensation of 

 this muscular movement undoubtedly plays an important part in 

 giving us the idea of depth or solidity. For persons not practised 

 in the matter of observing stereoscopic pictures the full idea of relief 

 comes out only after this muscular activity has been called upon. 

 But for the practised eye this play of the muscles is not absolutely 

 necessary. The stereoscopic picture stands out in relief even when 

 illuminated momentarily by the light of an electric spark. The per- 

 ception of solidity in this case is instantaneous, and it has been sug- 

 gested that this result may depend upon the immediate recognition 

 of physiological diplopia, that is, the fact that objects nearer than 

 the point of fixation are doubled heteronymously, while those 

 farther away are doubled homonymously (see p. 342). Such an 

 effect can only be produced distinctly by objects having depth 

 and possibly in the case of the trained eye it alone is sufficient to 

 give the immediate inference of solidity or relief, while the un- 

 trained eye requires the accessory sensations aroused by focal 

 adjustment, mathematical perspective, etc. 



Judgments of Distance and Size. Judgments of distance 

 and size are closely related. Our judgments regarding size are 

 based primarily upon the size of the retinal image, the amount of 

 the visual angle. This datum, however, is sufficient in itself only 

 for objects at the same distance from us. If they are at different 

 distances or we suppose that such is the case, our judgment of the 

 distance controls our judgment of size. This fact is beautifully 



