BINOCULAK VISION. 781 



increase their apparent height, and very stout persons avoid longitudinal 

 stripes. Two perfectly parallel lines or bands, each of which is crossed by 

 slanting parallel short lines, will appear not parallel, but diverging or con- 

 verging according to the direction of the cross-lines. 



Again, when a short person is placed side by side with a tall person, the 

 former appears shorter and the latter taller than each really is. The moon 

 on the horizon appears larger than when at tho zenith, because in the first 

 position it can be most easily compared with terrestrial objects. The absence 

 of comparison may, however, contribute to an opposite effect, as when a 

 person looks larger in a fog; being seen indistinctly, he is judged to be 

 further off than he really is, and so appears larger than he naturally would 

 do at the distance at which he is supposed to be. So, conversely, distant 

 mountains, when seen distinctly in a clear atmosphere appear small, because, 

 on account of their distinctness, they are judged to be nearer than they really 

 are. Indeed, our daily life is full of instances in which our direct percep- 

 tion is modified by circumstances. Among those circumstances previous 

 experience is one of the most potent, and thus simple perceptions become 

 mingled with what are in reality judgments, though frequently made uncon- 

 sciously. But this intrusion of past experience into present perceptions and 

 sensations is most obvious in binocular vision, to which we now turn. 



BINOCULAR VISION. 



Corresponding or Identical Points. 



680. Though we have two eyes, and must therefore receive from every 

 object two sets of sensations, our perception of an object is under ordinary 

 circumstances a single one ; we see one object, not two. But putting either 

 eye into an unusual position, as by squinting, we can render the perception 

 double ; we see two objects where one only exists. From which it is evi- 

 dent that singleness of perception depends on the image of the object fall- 

 ing on certain parts of each retina at the same time, these parts being so 

 related to each other that the sensations from each are blended into one per- 

 ception ; and it is also evident that the movements of the eyeballs are 

 adapted to bring the image of the object to fall on these " corresponding " 

 or " identical " parts, as they are called, of each retina. 



When we look at an object with one eye the visual axis of that eye is 

 directed to the object, and when we use two eyes the visual axes of the two 

 eyes converge at the object, the eyeballs moving accordingly. The corre- 

 sponding points of the two retinas are those on which the two images of the 

 object fall when the visual axes converge at the object. Thus in Fig. 179, 

 if xx, xx'be the two visual axes, xx' being the centres of the fovese centrales 

 of the two eyes, then, the object y, x, z being seen single, the point y on the 

 one retina will " correspond " to or be " identical " with the point y' on the 

 other, and the point x in the one to the point x' in the other. Hence a point 

 lying anywhere on the right side of one retina has its corresponding point 

 on the right side of the other retina, and the points on the left of one corre- 

 spond with those on the left of the other. Thus, while the upper half of the 

 retina of the left eye corresponds to the upper half of the retina of the right 

 eye and the lower to the lower, the nasal side of the left eye corresponds 

 with the malar side of the right, and the malar of the left* with the nasal 

 side of the right. 



The blending of the two sensations into one only occurs when the two 

 images of an object fall on these corresponding points of the two retinas. 

 Hence it is obvious that in single vision with two eyes the ordinary move- 



