BINOCULAR VISION. 



347 



nature or by an artifice, we at once perceive depth or solidity in the 

 object. This fact is made use of in all devices employed to produce 

 stereoscopic vision. 



Stereoscopic Vision. Stereoscopic pictures may be obtained 

 by photographing the 

 same object or collec- 

 tion of objects from 

 slightly different 

 points so as to get a 

 right-eyed and a left- 

 eyed picture ; or for 

 simple outline pic- 



Fig. 150. Right- and left-eyed images of truncated 

 pyramid. May be combined to produce solid image by 

 relaxing the accommodation, that is, gazing to a dis- 

 tance through the book. 



tures, such as geo- 

 metrical figures, they 

 may be made by draw- 

 ings of the object as seen by the two eyes, respectively (see Figs. 

 150 and 152). Any optical device that will enable us to throw the 

 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; 

 the one that is most frequently 

 used is the Brewster stereoscope 

 represented in principle in Fig. 

 151. 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 



princi 

 (Lana 



151. Diagram to illustrate the 

 of the Brewster stereoscope 

 3) : P and P', the prisms, o, &, 

 and a, 0, the left- and right-eyed pictures, 

 respectively, 6, ft, 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. 



