EXPERIMENTS OX DOUBLE VISION*. 213 



the images on the two retinas, that the habit of judging Hence a single 

 an object seen double to be single is established ; an object double^hea* 

 will appear double, whenever this correspondence docs not the correspou- 

 take place. This happens when we press upon one off prfessionsonAe 

 the eyes in such a manner, as to determine the impression eyes is altered, 

 of the image of a single object to parts of the two retinas 

 that are not commonly acted unon simultaneously. Xhis 1 1(? " ewcor " 



J . J respondeiice 



happened, as Cheseldcn relates, to a man who had one of is learned from 

 his eyes distorted by a blow ; and he saw objects double, !aoit " 

 till habit and experience had done for the new points of 

 correspondence on the two retinas, what they had before 

 effected for the points that corresponded previous to the ac- 

 cident. 



Double objects will appear single on the contrary, when, F f encp clouwe 



. . . . , . , , . , , objects appear 



their impression being made separately and simultaneously single by means 



on corresponding points of the retinas, we experience the ° f th ^ s corres - 



same impressions, as would be excited in us by the picture 



of a single object. This is Avhat takes place in our expe- as in the P re * 

 r i • r i • . ceding expe- 



riments : for the two eyes, in consequence of the vertical nments. 



plane that separates them, receiving separately different 

 and simultaneous impressions on points of the retinas which 

 are the same, or nearly the same, as those that correspond 

 by habit to the two images of a single object, must excite 

 in us the idea of the presence of a single object only. What 

 convinces me that this association or combination of the 

 double image depends on its being depicted on correspond- 

 ing points of the two retinas is, that, when this corres- 

 pondence is destroyed by placing the objects at too great a 

 distance from each other, or by giving them too great extent, 

 the association does not take place, and we have a distinct 

 perception of the images of both objects. This in fact 

 must be the case, for the rays proceeding from one object 

 to both eyes necessarily observe certain proportions of cor- 

 respondence, which cannot exceed a given limit. 



The superposition and apparent motion of the two objects The motion of 

 one toward the other is an illusion produced by the force tow ^J e ^i, 

 of habit, which, having constantly taught us, that objects, other an illu- 

 the double image of which is painted on the bottom of our sl0n ' 

 eyes, without exciting a double perception, are placed in 

 the point of meeting of the visual rays, transfers them to 



the 



