VIII 



OCULAR MOVEMENTS 



41 :; 



it consists of two mirrors, M, J\J, arranged at an angle directed 

 towards the median plane of the observer. The two perspective 

 drawings for instance, that of the truncated pyramid as above 



C 



I' 



1. R 



l'i. I'.'iv --l)i;i^rain of Wheatstoiie's mirror stereoscope. 



are arranged so that the image R is reflected into the right eye 

 R, the image L' into the left eye. In consciousness the different 

 points of the two images fuse, 

 although they fall to a great 

 extent on non-corresponding 

 points of the retinae by being 

 successively or simultaneously 

 superposed, and thus the trun- 

 cated pyramid is seen in 

 perfect relief. 



Brewster's stereoscope 

 (1849) is more generally used. 

 It consists of two biconvex 

 prisms PP(Fig.l99),separated 

 by a vertical plane (pp'). 

 Owing to refraction by the 

 two prisms the two images I'r' 

 are superposed into a single 

 complete image ((7(7') in the 

 plane on which the two visual 

 axes converge. 



It is also possible, without 

 employing any instrument, to 

 fuse two suitable images into 

 one. It is only necessary to 

 keep the two visual axes 

 parallel, that is, to fixate point , ,,, Diagram of Brewstl ,, s pri8m stereoscope. 

 a with the right eye, point a' 



with the left; three images will then be seen, each of the lateral 

 with the eye of the corresponding side, while the middle image 

 is seen with both eyes. The two lateral images are flat and two- 

 dimensional, while the third is in relief and three-dimensional. 



