414 



PHYSIOLOGY 



CHAP. 



To facilitate vision in relief without a stereoscope it is as well to 

 interpose a septum between the two drawings (like tt' in Brewster's 

 stereoscope) by which the lateral images are cut out. The stereo- 

 scope is therefore not indispensable, but it brings about the fusion 

 of the two images without any fatigue of accommodation, and with 

 remarkable precision, while the eyes are in the ordinary position. 



By means of photography we can prepare two perspective 

 images of any figure, scene, or landscape for stereoscopic vision. 

 The two photographs are taken with twin cameras, the two lenses 

 being fixed at the same distance apart as the two eyes, viz. 60-65 

 mm. Experience, however, teaches that a better stereoscopic effect 

 is produced by placing the two lenses farther apart (e.g. 70-75 mm.) 

 the background then appears deeper, and figures and objects seem 

 nearer than the distance at which they were photographed. 



In addition to the fusion of the two perspective images in the 

 stereoscope, there is very often an effect known as stereoscopic lustre, 

 due, as stated above, to the conflict of the two uniocular sensations. 



FIG. 200. 

 , other black 



Perspective images of two truncated pyramids with quadrangular bases, one white, the 

 T black, which are combined by binocular rivalry into a lustrous stereoscopic image. 



If, for instance, we look through the stereoscope at two truncated 

 pyramids (Fig. 200), one white with black outlines, the other 

 black with white outlines, they fuse into a pyramid in high relief 

 of lustrous grey, as though cut out of graphite, with illuminated 

 sides. 



If the picture of the left-hand pyramid is placed on the right 

 side, and that of the right-hand pyramid on the left, an inverted 

 relief appears, that is the view of a hollow pyramid seen from 

 below. Such images are known as pseudoscopic. The same effect 

 is produced by reversing the position of the double photographs 

 taken for the stereoscope. This is easy to explain, since the figures 

 which give the impression of a hollow pyramid only differ from 

 those which appear as a solid pyramid because the right eye is 

 looking at the figure usually seen with the left eye, and vice versa. 



Symmetrical objects can be seen in inverted relief with Wheat- 

 stone's pseudoscope, which consists of two right-angled prisms, 

 arranged as in the diagram (Fig. 201). On looking with both 

 eyes at a solid symmetrical body pseudoscopic vision is obtained. 

 Owing to reflection from the surface of the hypothenuse of the 



