MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 393 



for instance red lines on a green ground, and be viewed either in the stereoscope or in 

 the apparatus, fig". 6., as the ordinary figures are, taking care however to fix the eyes 

 only to a single point of the compound figure; the drawings must be strongly illu- 

 minated, and after a sufficient time has elapsed to impress the spectra on the retinse, 

 the eyes must be carefully covered to exclude all external light. A spectrum of the 

 object in relief will then appear before the closed eyes. It is well known, that a spec- 

 trum impressed on a single eye and seen in the dark, frequently alternately appears 

 and disappears : these alternations do not correspond in the spectra impressed on the 

 two retinae, and hence a curious effect arises ; sometimes the right eye spectrum will 

 be seen alone, sometimes that of the left eye, and at those moments when the two 

 appear together, the binocular spectrum will present itself in bold relief. As in this 

 case the pictures cannot shift their places on the retinse 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 termi- 

 nate at corresponding points of the two retinse 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 intersection of the lines of visible direction in 

 which it is seen by each eye singly, we should be in error. On this supposition, ob- 

 jects 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-corre- 

 sponding points occurs and is limited. I have made numerous experiments for the 

 purpose of attaining this end, and have ascertained some of the conditions on which 

 single and double vision depend, the consideration of which however must at present 

 be deferred. 



Sufficient, however, has been shown to prove that the laws of binocular visible di- 

 rection hitherto laid down are too restricted to be true. The law of Aguilonius as- 

 sumes that objects in the plane of the horopter are alone seen single ; and the law of 

 corresponding points carried to its necessary consequen(;es, though these conse- 

 quences were unforeseen by its first advocates, many of whom thought that it was 

 consistent with the law of Aguilonius, leads to the conclusion, that no object appears 

 single unless 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 incon- 

 sistent with the single vision of objects whose points lie out of the plane in one case 

 and the circle in the other ; and that objects do appear single under circumstances 



MDCCCXXXVIII. 3 B 



