BINOCULAR VISION. 359 



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. 152). 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. 352). 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 

 shown in the case of after-images (see p. 332). When an after- 

 image of any object is obtained on the retina our judgment of its 

 size depends altogether on the distance to which we project it. 

 If we look at a surface near at hand, it seems small ; if we gaze at a 

 wall many feet away it is at once greatly enlarged. The familiar 

 instance of the variation in the size of the full moon according as it 

 is seen at the horizon or at the zenith depends upon the same fact. 

 The distance to the horizon as viewed along the surface of the earth 

 seems greater than to the zenith; we picture the heavens above us 

 as an arched dome flattened at the top, and hence the same size of 

 retinal image is interpreted as larger when we suppose that we see 

 it at a greater distance. Our judgments of distance, on the other 

 hand, depend primarily upon the data already enumerated in 

 speaking of the perception of solidity or depth in the visual field. 



