HERMANN VON HELMHOLTZ 



horopter is really a horizontal plane passing through 

 our feet, thus warranting the name first given to it by 

 Aguilonius in 1613. The horopter in this case is the 

 ground on which we stand. Experiments show 

 1 that the forms and the distances of these objects 

 which are situated in, or very nearly in, the horopter, 

 are perceived with a greater degree of accuracy than 

 the same forms and distances would be when not situ- 

 ated in the horopter.' 



The investigation of this subject led Helmholtz to 

 the invention of the telestereoscope, an instrument 

 containing a combination of prisms, by which two 

 images of distant objects can be seen as if the eyes 

 were widely separated in the head. Consequently, 

 combination takes place, and the objects are seen in 

 relief. Further, Helmholtz, in his later researches, 

 came to the conclusion, that the apparent fusion 

 of two retinal images cannot be explained by 

 any anatomical arrangement, but that it is due 

 to a mental act. Briicke's notion, that the per- 

 ception of solidity might be due to sensations excited 

 by muscular contractions causing convergence of the 

 visual axes, is negatived by Dove's observation, 

 that the illusion of stereoscopic pictures is also pro- 

 duced when they are illuminated by the electric 

 spark, lasting less than the f^fth of a second, 

 and in 'this short time there cannot be the 

 slightest movement of the eyeballs. The study of 



stereoscopic lustre, also first noticed by Dove, pro- 

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