212 EXPERIMENTS ON DOUBLE VISION. 



duced into them, so that the sounds excited in one of the 

 organs were kept distinct from those excited in the other. 

 Sounds inpres- On making the strings of these little in'struments vibrate at 

 ear separately" tllc same t ' me > * convmce( l myself, that the different im- 

 eombiue. pressions produced by different sounds combined in the 



same manner, as when they are received simultaneously by 

 the same ear. The monocords tuned to thirds, fourths, or 

 fifths, to each other, produced the perceptions correspond- 

 ing with those concords. I know not whether it were 

 prejudice, but these concords seemed to me better united, 

 and more harmonious, that when received by the same 

 ear. 



These facts, and many others of a similar nature, leave 

 no doubt, that we enjoy the faculty of identifying or com- 

 bining heterogeneous sensations. However, as we can 

 likewise experience a distinct perception of heterogeneous 

 This faculty and simultaneous impressions, it is evident, that this fa- 

 circumsunces. Cl 'lty iS not so inherent in our organization, as to be inde- 

 pendent of certain circumstances, without which we cannot 

 Instance in produce the effects that characterise it. Thus we cease to 

 experience the perception that results from the apparent 

 combination of heterogeneous colours, when the objects are 

 at too great a distance from the vertical plane that separates 

 them, or of too great extent. 

 We see objects These circumstances well considered have enabled me to 

 •because our reduce these facts to the common laws of vision, 'accord- 

 judgment is ing to the theory of Buffon and Condillac, which is that of 

 touch 10 J the majority of physiologists. Objects according to these 

 philosophers appear to us single, though there is a double 

 representation of them, one at the bottom of each eye, 

 because the touch, which corrects the judgment formed 

 by the sight, teaches us, that the object, which we see 

 afterward from double, is notwithstanding single. Habit and expert* 

 k ablt - ence have rendered this manner of seeing so necessary, 



that it is impossible for us to change, without disturbing 

 the order established between the sight and touch. But 

 as every object, that produces a double impression on the re- 

 tina, is necessarily in the point of meeting of the optic axes, t 

 and consecpicntiy painted on corresponding points of the 

 retina j and that it is with respect to this correspondence of 



the 



