Dr. W. von Bezold on Binocular Vision. 331 



Finally there remains the question whether stereoscopic pheno- 

 mena accord with the assumption. Before entering on an inves- 

 tigation of this point, we must be clear as to the meaning to 

 be assigned to the words Sensation (Empfindung), Perception 

 (Wahrnehmung) , and Conception or Idea (Vorstellung) ; for an 

 intermixture of these ideas has produced great confusion. 



The objects surrounding us produce on our senses impressions 

 which come to our consciousness as sensations. By frequently 

 meeting the same objects, we always perceive the same sums of 

 sensations. Every such sum of sensations is in the course of 

 time apprehended by the memory as a connected whole; and 

 thus we attain to conceptions of things, to which latter certain 

 signs are attached as. characteristics. Meeting an object after 

 the formation of a conception, the sum of sensations is, without 

 further mental activity, directly combined with the conception to 

 form a perception. Sensations are conceivable without simulta- 

 neous conceptions referring to them ; conceptions without simul- 

 taneous corresponding sensations; perceptions, on the contrary, 

 require both simultaneously. 



If an object be looked at with both eyes, or we look into 

 a stereoscope, not merely is there formed a conception of 

 the object (for a linear drawing would be sufficient for this, 

 or even a description), but the sensuous impression imme- 

 diately evokes the conception in so cogent a manner that both 

 come simultaneously to the consciousness as a single perception. 

 We not merely represent to ourselves the body, and we have not 

 merely an optical sensation, but we see the body in question. The 

 case is similar with the perceptions of touch. If a body of 

 known kind be touched in any way, held in the hand, we are 

 no longer conscious of the individual sensations, but we at once 

 perceive a rope, a bar, a billiard-ball, &c. as such, even if no 

 other sense comes to aid that of touch. 



This being premised, the fact that the images projected in 

 both eyes are blended to a single perception of solidity appears 

 by no means strange ; and after such considerations no one will 

 consider it necessary to look for an anatomical connexion be- 

 tween so-called identical points. Why should not a single per- 

 ception be thus produced just as well as it is produced by the 

 two parts of the skin of adjacent fingers, which, turned to one 

 another, touch one and the same object ? 



From the point of view thus established, we need for ex- 

 plaining the perception of solidity by binocular vision the single 

 assumption only, that the impression which the excitation of two 

 adjacent parts of the double retina produces depends, under all 

 circumstances, on the mutual position of these places, even if they 

 are so near that the perception produced is single. If we only 



