Dr. W. von Bezold on Binocular Vision. 327 



an "empirical horopter," by which is understood the aggregate 

 of all points which, in accordance with experience, are observed 

 single. The fact just adduced shows that the principle of iden- 

 tity is not strictly valid. To bring it into unison with the prin- 

 ciple, it was assumed that in vision the attention is exclusively 

 devoted to the object fixed, and therefore the double images 

 which occur by the way, being simply overlooked, were neglected. 

 In spite of this weak part, and of the unsuccessful attempts 

 to discover by means of the microscope a connexion of the nerve- 

 fibres leading to identical points, and thus give the theory an 

 anatomical basis, it kept its ground for a long time, and during 

 a series of years served as the sole starting-point for all investi- 

 gations upon this subject. 



Views on this matter have taken a different shape since 

 Wheatstone, by his ingenious discovery of the stereoscope, 

 showed that it is just the difference in the two images on the 

 retina which forms the chief condition for the production of the 

 perception of solidity. Stereoscopic experiments showed that 

 simultaneous excitation of non-identical points can produce 

 single perceptions of a special kind which are by no means ex- 

 plained by simply neglecting the double images. 



As the principle of identity in its rigid form was thus shaken, 

 it was endeavoured to overthrow it completely, and trouble was 

 taken to prove that it is possible to see double even with 

 identical points*. Yet, according to E. Hering's investigations, 

 all the attempts in this direction must be regarded as complete 

 failures. The first of my above-mentioned papers had the object of 

 adducing further proofs of the untenability of this proposition. 



It was now incumbent on the opponents of the theory of 

 identity to replace by another the principle which they had 

 rejected. They did this by making the following assumption: — 

 The cause of the excitation which a spot in the retina ex- 

 periences is involuntarily sought in the straight line joining 

 the excited spot with the object; that is, it is transported, 

 "projected" outwards, and, indeed, in a certain surface which 

 passes through the fixed point and is called the surface of 

 projection-]-. If this proposition be admitted, it follows that only 

 those points which lie in this surface are seen single ; all objects 

 which are nearer, and all which are more distant, must appear 

 double, because the corresponding lines of projection cut the 



* See Wundt in Henle and Pfeufer, Zeitschrift fur ration. Medicin, 

 ser. 3. vol. xii. p. 249. Further, A. Nagel, Das Sehen mit zwei Augen : 

 Leipzig, 1861. 



t A. Nagel, who has most developed this doctrine, speaks of two sur- 

 faces ; yet these lie in general so near each other, that for the present 

 consideration they may just as well be supposed to be replaced by a 

 single one. 



