STEREOSCOPIC VISION. 789 



Neglect of Double Images. Notwithstanding the very large number of double 

 images which must be formed during vision, they do not disturb vision. As a 

 general rule they are " neglected," so that the attention must, as a rule, be directed 

 to them before they are perceived. This condition is favoured thus : 



1. The attention is always directed to the point of the field of vision which is accommodated 

 for at the time. The image of this part is projected on to both yellow spots, which are 

 identical points of the retina. 



2. The form and colour of objects on the lateral parts of the retina are not perceived so 

 sharply. 



3. The eyes are always accommodated for those points which are looked at. Hence, 

 indistinct images with diffusion circles are always formed by those objects which yield double 

 images, so that they can be more readily neglected. 



4. Many double images lie so close together that the greater part of them, when the images 

 are large, covers the other. 



5. By practice images which do not exactly coincide may be united. 



402. STEREOSCOPIC VISION. On looking at an object, both eyes do not 

 yield exactly similar images of that object the images are slightly different, 

 because the two eyes look at the object from two differ- * 



ent points of view. With the right eye we can see 

 more of the side of the body directed towards it, and 

 the same is the case with the left eye. Notwithstand- 

 ing this inequality, the two images are united. How 

 two different images are combined is best understood 

 by analysing the stereoscopic images. jj jj jj~ 



Let, in fig. 569, L and R represent two such images as are L R 



obtained with the left and right eyes. These images, when Fig. 569. 



seen with a stereoscope, look like a truncated pyramid, which Two stereoscopic drawings, 

 projects towards the eye of the observer, as the points indicated 



by the same signs cover each other. On measuring the distance of the points, which coincide 

 or cover each other in both figures, we find that the distances A, a, B, b, C, c, D, d are equally 

 great, and at the same time are the widest of all the points of both figures ; the distances E, e, 

 F, /> G> g, H, h are also equal, but are smaller than the former. On looking at the coincid- 

 ing lines (A, E, a, e, and B, F, b, f), we observe that all the points of this line which lie nearer 

 to A a and B b are further apart than those lying nearer E e and F/. 



Comparing these results with the stereoscopic image, we have the following laws 

 for stereoscopic vision: 1. All those points of two stereoscopic images, and of 

 course of two retinal images of an object, which in both images are equally distant 

 from each other, appear on the same plane. 2. All points which are nearer to each 

 other, compared with the distance of other points, appear to be nearer to the 

 observer. 3. Conversely, all points which lie further apart from each other appear 

 perspectively in the background. 



The cause of this phenomenon lies in the fact that, " in vision with both eyes 

 we constantly refer the position of the individual images in the direction of the 

 visual axis to where they both intersect." 



Proofs. The following stereoscopic experiment proves this (fig. 570) : Take both images of 

 two pairs of points (a, b, and a, )8), which are at unequal distances from each other on the 

 surface of the paper. By means of small stereoscopic prisms cause them to coincide, then the 

 combined point, A of a, and o appears at a distance on the plane of the paper, while the other 

 point, B, produced by the superposition of b and 0, floats in the air before the observer. Fig. 

 570 shows how this occurs. The following experiment shows the same result : Draw two 

 figures, which are to be superposed similar to the lines B, A, A, E, b, a, and a, e, in fig. 569. 

 In the lines B, A, and b, a, all the points which are to be superposed lie equally distant from 

 each other, while, on the contrary, all the points in A, E, and a, e, which lie nearer E and e, 

 are constantly nearer to each other. When looked at with a stereoscope, the superposed 

 verticals, A, e, and B, b, lie in the plane of the paper, while the superposed lines, A, a, and E, 

 e, project obliquely towards the observer from the plane of the paper. From these two 

 fundamental experiments we may analyse all pairs of stereoscopic pictures. Thus, in fig. 569, 

 if we exchange the two pictures, so that R lies in the place of L, then we must obtain the 

 impression of a truncated hollow pyramid. 



