STEREOSCOPIC VISION. JUDGMENT OF SOLIDITY. 873 



In spite of the large number of double images that are constantly formed, 

 they are not a source of disturbance. They are usually suppressed, and to such 

 an extent, in fact, that the attention must be directed to them, in order that 

 they may be seen. The suppression of the double images is favored by the follow- 

 ing factors: (i) Attention is always directed to that point of the visual field 

 which for the time being is fixed. This throws its image on the two yellow spots, 

 which are identical points. (2) Form and color are less sharply seen by the lateral 

 portions of the retina. (3) The eyes are always accommodated for those points 

 that are fixed. Therefore, only indistinct images arise from the objects that 

 produce double images (in diffusion-circles), and these can be more easily sup- 

 pressed. (4) Many double images lie so close together that when they are large, 

 the greater portions of them overlap. (5) Images that, strictly speaking, do not 

 coincide are often united by a psychical habit. 



STEREOSCOPIC VISION. JUDGMENT OF SOLIDITY. 



The images formed by the two eyes in looking at solid objects are not 

 exactly alike, but differ somewhat, because the eyes look at the ob- 

 ject from two different points of view. The right eye can see more of 

 the side opposite to it, and the same is true of the left eye. Despite this 

 dissimilarity, the two images are united. 



The question as to how the impression of solidity is obtained by the 

 combination of such different images may be best solved by analyzing 

 two corresponding stereoscopic pictures. 



In Fig. 305, III, L and R are two such pictures that, when seen with a stereo- 

 scope, form a truncated pyramid, projecting toward the eye of the observer, and 

 the similarly designated points coincide. If the distances between the corre- 

 sponding points in the two figures be measured, it will be found that the distances 

 A a, B b, C c, D d are equal, and are at the same time the greatest between any 

 of the points of the two figures; further, the distances E e, F f, G g, H h are equal, 

 but are smaller than the first set. Considering, finally, the lines A E, a e, and 

 B F, b f, which coincide, it may easily be seen that all points of these lines that 

 lie nearer A a and B b are further apart than those that lie nearer E e and F f. 



From a consideration of these relations in comparison with the 

 stereoscopic images the following principles for stereoscopic vision 

 appear: (i) All those points of two stereoscopic images (and naturally 

 of two retinal images of solid objects) that are at equal distances from 

 each other in the two images appear in the same plane. (2) All points 

 that are closer together (than the others) project toward the observer. 

 (3) Conversely, the points that are further apart recede perspectively 

 into the background. 



The reason for this phenomenon resides simply in the following 

 principle: "In binocular vision we constantly refer the position of the 

 individual points of the image in the direction of the visual axes where 

 they intersect." 



The following stereoscopic experiment proves this. In Fig. 305 I, two pairs 

 of points are taken as the two images (a b and ft} that are at unequal distances 

 from each other on the surface of the paper. If they are made to coincide, by 

 means of a stereoscope, the point (A), formed by the union of a and a, appears in 

 the plane of the paper, while the other (B) formed by the points b and p, which 

 are closer, seems to float in the air in front of A. Fig. 305, I, shows the construc- 

 tion clearly. The following experiment also illustrates the same condition. Two 

 sets of lines, similar to B A, A E and b a, a e in Fig. 305, III, are drawn as the 

 figures to be superposed. In the lines B A and b a all the points to be superposed 

 lie equally distant from each other; on the contrary, all points in A E and a e, 

 which lie nearer to E and e, are successively closer to each other. Looked at 

 with a stereoscope, the superposed perpendicular line A B and a b lies in the plane 

 of the paper, while the oblique line formed by A E and a e projects obliquely 



