Prof. Wheatstone on the Physiology of Vision. 267 



retiuse in whatever manner the eyes be moved about, the optic 

 axes can during the experiment only correspond with a single 

 point of each. 



When an object, or a part of an object, thus appears in relief 

 while the optic axes are directed to a single binocular point, it 

 is easy to see that each point of the figure that appears single is 

 seen at the intersection of the two lines of visible direction in 

 which it is seen by each eye separately, whether these lines of 

 visible direction terminate at corresponding points of the two 

 retinae or not. 



But if we were to infer the converse of this, viz. that every 

 point of an object in relief is seen by a single glance at the inter- 

 section of the lines of visible direction in which it is seen by each 

 eye singly, we should be in error. On this supposition, objects 

 before or beyond the intersection of the optic axes should never 

 appear double, and we have abundant evidence that they do. 

 The determination of the points which shall appear single seems 

 to depend in no small degree on previous knowledge of the form 

 we are regarding. No doubt, some law or rule of vision may be 

 discovered which shall include all the circumstances under which 

 single vision by means of non-corresponding points occurs and 

 is limited. I have made numerous experiments for the purpose 

 of attaining this end, and have ascertained some of the con- 

 ditions on which single and double vision depend, the conside- 

 ration of which however must at present be deferred. 



Sufficient, however, has been shown to prove that the laws of 

 binocular visible position hitherto laid down are too restricted 

 to be true. The law of Aguilonius assumes that objects in the 

 plane of the horopter are alone seen single ; and the law of cor- 

 responding points carried to its necessary consequences, though 

 these consequences were unforeseen by its first advocates, many 

 of whom thought that it was consistent with the law of Aguilo- 

 nius, leads to the conclusion, that no object appears single un- 

 1< s- it is seen in a circle passing through the centres of visible 

 direction in each eye and the point of convergence of the optic 

 axes. Both of these are inconsistent with the single vision of 

 objects whose points lie out of the plane in one case and the 

 circle in the otln rj and that objects do appear single under cir- 

 cumstances that cannot be explained by these laws, has, I think, 

 bicn placed beyond doubt by the experiments I have brought 

 forward. Should it be hereafter proved, that all points in the 

 plane or in the circle above mentioned are seen single, and from 

 the great indistinctness of lateral Images it will be difficult to 

 give this proof, the law must lie qualified by the admit sion, that 

 points out of them do not always appear double. 



