THE SENSE OF VISION 



635 



The effect of combining the pictures of the left and right eye is 

 well seen in the stereoscope. In the more commonly used form, the 

 Brewster stereoscope, the corresponding picture for each eye is viewed 

 through a curved prism, so placed with its base outwards that the 

 rays from the pictures impinge on the retina 

 in lines of convergence which meet at points 

 behind the plane of the pictures (see Fig. 351). 

 A partition cuts off the sight of the opposite 

 picture, so that each eye sees only its own 

 picture, usually slightly magnified by the prism. 

 Owing to differing distances between the corre- 

 sponding points in the two individual pictures, 

 the effect of apparently differing distances is 

 obtained in the combined picture. The nearest 

 points that is, those which require the greatest 

 convergence of the optic axes stand out in 

 most marked relief, the most distant points, 

 which require less convergence, in least relief. 

 It is not necessary to have a stereoscope to get 

 this combined effect. With practice, it is 

 accomplished by merely fixing the eyes on the 

 pictures, and adjusting the eyes for distant 

 vision. The two pictures then gradually merge 



together, and the whole effect of solidity is suddenly perceived. 

 This is known as haploscopic vision (Fig. 362). 



Stereoscopic pictures are obtained by drawing or photographing 

 the same scene from slightly different points of view. The perception 

 of solidity is due in the main to a mental process fusing slightly 



A 8 



FIG. 361. DIAGRAM OF 

 A STEREOSCOPE. 



Two photographs, A 

 arid B, are seen at C. 

 The rays of light from 

 A and B are refracted 

 by the prisms into the 

 eyes so that they 

 appear to come 

 from 0. 



FIG. 362. To ILLUSTRATE HAPLOSCDPIO VISION. 



The distance between the fish and tank is here given as rather less than the distance 

 between the two pupils, so that the haploscopic combination can ba effected 

 easily, with the visual axes somewhat convergent. The illusion is facilitated by 

 holding a card vertically between the eyes, so as to hide the fish from tin left 

 eye and th3 tank from the right eye. 



different images from the two retinae, the amount of activity of the 

 ye muscles in converging the optic axes being also an important 

 factor. 



Judgment of size and distance also depends largely upon the 

 <legree of muscular effort put forth in the convergence of the eyes 



