VIII 



OCULAR MOVEMENTS 



415 



prisms, the rays from a more distant object fall on the temporal 

 half of the retina, and not on the nasal side as they usually do ; 

 in other words, the right eye receives the image which would 

 normally fall on the left retina, and vice versa. The fusion of these 

 two images results in complete inversion of the relief of the 

 object. 



The same effect is obtained with R. Ewald's katoptric pseudo- 

 scope, in which the inversion of the rays to the two eyes is 



Fi'.. 201. Diagram of Wheat stone's 

 pseudoscope. 



L R 



c. 202. Diagram of U. Ewald's 

 katopl i i<- (isruiloscope. 



obtained by reflection from two pairs of mirrors, arranged as in 

 Fig. 202. 



We do not see far-distant objects stereoscopically, because the 

 difference between the two fixation points is too insignificant to 

 modify the uniocular images perceptibly. Helmholtz succeeded, 

 by means of his telestereoscope, in magnifying the distance 

 between the two eyes artificially, so that he was able to see 

 landscapes on a distant hori/ou in relief. As shown by Tig. 

 203 his telestereoscope is only a form of Wheatstone's mirror 

 stereoscope, in which the two drawings are replaced by two 

 mirrors, turned to the horizon, parallel with the internal mirrors. 

 Obviously, under these conditions, distant bodies are seen as if 



