376 



THE SPECIAL SENSES. 



right-eyed picture on the right eye and the left-eyed picture on 

 the left eye constitutes a stereoscope. Many different forms of 



stereoscope have been devised; 



A ^ the one that is most frequently 



:\ \ / used is the Brewster stereoscope 



I \ \''^ represented in principle in Fig. 



,' \ /\ 161. Each eye views its corre- 



sponding picture through a 

 curved prism. The sight of the 

 left-eyed picture is cut off from 

 the right eye, and vice versa, by 

 a partition extending for some 

 distance in the median plane. 

 The prisms are placed with their 

 bases outward and the rays of 

 light from the pictures are re- 

 fracted, as shown in the diagram, 

 so as to aid the eyes in converg- 

 ing their lines of sight upon the 

 same object. The prisms also 

 magnify the pictures somewhat. 

 Stereoscopic pictures are mounted 

 usually for this instrument so that 

 the distance between the same 

 object in the two pictures is about 80 mms. — greater, therefore, than 

 the interocular distance. A simple form of stereoscope that is very 

 effective and interesting is sold under the name of the anaglyph. 

 The two pictures in this case are approximately superposed, but the 

 outlines of one are in blue and the other in red. When looked at, 

 therefore, the picture gives an ordinary fiat view with confused 

 red-blue outlines. If, however, one holds a piece of red glass in 

 front of the left eye and a piece of blue glass in front of the right eye, 

 or more conveniently uses the pair of spectacles provided which 

 have blue glass on one side, red on the other, then the picture stands 

 out at once in soUd relief with surprising distinctness — and as a 

 black and white object only. The red and blue glasses in this case 

 simply serve to throw the right-eyed image on the right eye and the 

 left-eyed image on the left eye. Assuming that the right-eyed 

 image is outlined in red, then the blue glass should be in front of the 

 right eye. This glass will absorb the red rays completely so that 

 the red outlines in the picture will seem black and a distinct right- 

 eyed picture is thrown on the right eye, distinct enough to make us 

 overlook the much fainter image in blue, which is also trans- 

 mitted through the blue glass. The red glass before the left 



Fig. 161. — Diagram to illustrate the 

 principle of the Brewster stereoscope 

 (Landois) : P and P', the prisms, a, b, 

 and a, ^, the left- and rirjht-eyed pictures, 

 respectively, b, /3, being a point in the 

 foreground and a, a, a point in the back- 

 ground. The eyes are converged and 

 focused separately for each point as in 

 viewing naturally an object of three di- 

 mensions. 



