264 Prof. Wheatstone on the Physiology of Vision. 



with only one eye at a time though hoth remain open, one accord- 

 ing to them being relaxed and inattentive to objects while the 

 other is upon the stretch. It is a sufficient refutation of this 

 hypothesis, that we see an object double when one of the optic 

 axes is displaced either by squinting or by pressure on the eye- 

 ball with the finger ; if we saw with only one eye, one object 

 only should under such circumstances be seen. Again, in many 

 cases which I have already explained, the simultaneous affection 

 of the two retinas excites a different idea in the mind to that 

 consequent on either of the single impressions, the latter giving 

 rise to the idea of a representation on a plane surface, the former 

 to that of an object in relief; these things could not occur did 

 Ave see with only one eye at a time. 



Du Tour * bold that though we might occasionally see at the 

 same time with both eyes, yet the mind cannot be affected simul- 

 taneously by two corresponding points of the two images. He 

 was led to this opinion by the curious facts alluded to in § 14. 

 It would be difficult to disprove this conjecture by experiment ; 

 but all that the experiments adduced in its favoiu-, and others 

 relating to the disappearance of objects to one eye really proves, 

 is, that the mind is inattentive to impressions made on one 

 retina when it cannot combine the impressions on the two 

 retinae together so as to occasion a perception resembling that 

 of some external object ; but they afford no ground whatever 

 for supposing that the mind cannot under any circumstances 

 attend to impressions made simultaneously on points of the two 

 retinfe, when they harmonize with each other in suggesting to 

 the mind the same idea. 



A perfectly original theory has been recently advanced by 

 M. Lehot t, who has endeavoured to prove, that instead of pic- 

 tures on the retina?, images of three dimensions are formed in 

 the vitreous humour which we perceive by means of nervous 

 filaments extended thence from the retina. This theory would 

 account for the single appearance to both eyes of objects in re- 

 lief, but it would be quite insufficient to explain why we per- 

 ceive an object of three dimensions when two pictures of it are 

 presented to the eyes ; according to it, also, no difference should 

 be perceived in the relief of objects when seen by one or both 

 eyes, which is contrary to what really happens. The proofs, 

 besides, that we perceive external objects by means of pictures 

 on the retinas are so numerous and convincing, that a con- 

 trary conjecture cannot be entertained for a moment. On this 

 account it will suffice merely to mention two other theories. 



* Act. Par. 1743. M. p. 334. 



t Nouvelle Theorie de la Vision, Par. 1823. 



