Prof. Wheatstone on the Physiology of Vision. 265 



Vallee *, without denying the existence of pictures on the retina, 

 has advocated that we perceive the relief of objects by means of 

 anterior foci on the hyaloid membrane ; and Raspail f has deve- 

 loped at considerable length the strange hypothesis, that images 

 are not painted on the retina, but are immediately perceived at 

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



§16. 



It now remains to examine why two dissimilar pictures pro- 

 jected on the two retina? give rise to the perception of an object 

 in relief. I wall 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 com- 

 plexity. I shall in this place merely consider the most obvious 

 explanation which might be offered, and show its insufficiency to 

 explain the whole of the phenomena. 



It may be supposed, that we see but one point of an object 

 distinctly at the same instant, the one namely to which the 

 optic axes are directed, while all other points are seen so indi- 

 stinctly, that the mind docs not recognise them to be either 

 single or double, and that the figure is appreciated by success- 

 ively directing the point of convergence of the optic axes success- 

 ively to a sufficient number of its points to enable us to judge 

 accurately of its form. 



That there is a degree of indistinctness in those parts of the 

 field of view to which the eyes are not immediately directed, and 

 which increases with the distance from that point, cannot be 

 doubted, and it is also true that the objects thus obscurely seen 

 are frequently doubled. It may be said, this indistinctness and 

 duplicity is not attended to, because the eyes shifting continually 

 from point to point, every part of the object is successively ren- 

 dered distinct ; and the perception of the object is not the con- 

 sequence of a single glance, during which only a small part of it 

 is seen distinctly, but is formed from a comparison of all the 

 pictures successively seen while the eyes are changing from one 

 point of the object to another. 



All this is in some degree true ; but were it entirely so, no 

 appearance of relief should present itself when the eyes remain 

 intently fixed on one point of a binocular image in the stereo- 

 scope. But on performing the experiment carefully, it will be 

 found, provided the pictures do not extend too far beyond the 



* Traits de la Science du Densin, Par. 1821, p. 270. 

 t Nouveuu Systhne de Chhnie Oryanique, t. 2. p. 329. 



