MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 391 



the former to that of an object in relief; these things could not occur did we see 

 with only one eye at a time. 



Du Tour* held that though we might occasionally see at the same time with both 

 eyes, yet the mind cannot be affected simultaneously 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 expe- 

 riments adduced in its favour, and others relating to the disappearance of objects to 

 one eye really proves, is, that the mind is inattentive to impressions made on one re- 

 tina when it cannot combine the impressions on the two retinae together so as to 

 resemble the perception of some external objects ; but they afford no ground what- 

 ever for supposing that the mind cannot under any circumstances attend to im- 

 pressions made simultaneously on points of the two retinae, 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-}-, who has 

 endeavoured to prove, that instead of pictures on the retinae, images of three dimen- 

 sions are formed in the vitreous humour which we perceive by means of nervous fila- 

 ments extended thence from the retina. This theory would account for the single 

 appearance to both eyes of objects in relief, but it would be quite insufficient to ex- 

 plain why we perceive an object of three dimensions when two pictures of it are pre- 

 sented to the eyes ; according to it, also, no difference should be perceived in the re- 

 lief of objects when seen by one or both eyes, which is contrary to what really hap- 

 pens. The proofs, besides, that we perceive external objects by means of pictures on 

 the retinae are so numerous and convincing, that a contrary conjecture cannot be en- 

 tertained for a moment. On this account it will suffice merely to mention two other 

 theories which place the seat of vision in the vitreous humour. Vallee J, without 

 denying the existence of pictures on the retina, has advocated that we see the relief 

 of objects by means of anterior foci on the hyaloid membrane; and Raspail^ has 

 developed at considerable length the strange hypothesis, that images are neither 

 formed in the vitreous humour nor painted on the retina, but are immediately per- 

 ceived at the focus of the lenticular system of which the eye is formed. 



§ 16. 



It now remains to examine why two dissimilar pictures projected on the two re- 

 tinae give rise to the perception of an object in relief. I will not attempt at present 

 to give the complete solution of this question, which is far from being so easy as at 

 a first glance it may appear to be, and is indeed one of great complexity. I shall in 

 this place merely consider the most obvious explanations which might be offered, and 

 show their insufficiency to explain the whole of the phenomena. 



* Act. Par. 1743. M. p. 334. f Nouvelle Thdorie de la Vision, Par. 1823. 



X Traits de la Science du Dessein, Par. 1821, p. 270. 

 § Nouveau Syst^me de Chimie Organique, t. 2. p. 329. 



