388 MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 



namely the centre of its spherical surface. This one point may be called the centre 

 of visible direction, because every point of a visible object vrill be seen in the direc- 

 tion of a line drawn from this centre to the visible point." 



It is obvious, that the result of any attempt to explain the single appearance of ob- 

 jects to both eyeSj or, in other words, the law of visible direction for binocular vision, 

 ought to contain nothing inconsistent with the law of visible direction for monocular 

 vision. 



It was the opinion of Aguilonius, that all objects seen at the same glance with both 

 eyes appear to be in the plane of the horopter. The horopter he defines to be a line 

 drawn through the point of intersection of the optic axes, and parallel to the line 

 joining the centres of the two eyes ; the plane of the horopter to be a plane passing 

 through this line at right angles to that of the optic axes. All objects which are in 

 this plane, must, according to him, appear single because the lines of direction in 

 which any point of an object is seen coincide only in this plane and nowhere else ; 

 and as these lines can meet each other only in one point, it follows from the hypo- 

 thesis, that all objects not in the plane of the horopter must appear double, because 

 their lines of direction intersect each other, either before or after they pass through 

 it. This opinion was also maintained by Dechales and Porterfield. That it is 

 erroneous, I have given, I think, sufficient proof, in showing that, when the optic axes 

 converge to any point, objects before or beyond the plane of the horopter are under 

 certain circumstances equally seen single as those in that plane. 



Dr. Wells's " new theory of visible direction" was a modification of the preceding 

 hypothesis. This acute writer held with Aguilonius, that objects are seen single only 

 when they are in the plane of the horopter, and consequently that they appear double 

 when they are either before or beyond it ; but he attempted to make this single ap- 

 pearance of objects only in the plane of the horopter to depend on other principles, 

 from which he deduced, contrary to Aguilonius, that the objects which are doubled 

 do not appear in the plane of the horopter, but in other places which are determined 

 by these principles. Dr. Wells was led to his new theory by a fact which he acci- 

 dentally observed, and which he could not reconcile with any existing theory of 

 visible direction ; this fact had, though he was unaware of it, been previously noticed 

 by Dr. Smith ; it is already mentioned in § 8., and is the only instance of binocular 

 vision of relief which I have found recorded previous to my own investigations. So 

 little does Dr. Wells's theory appear to have been understood, that no subsequent 

 writer has attempted either to confirm or disprove his opinions. It would be useless 

 here to discuss the principles of this theory, which was framed to account for an ano- 

 malous individual fact, since it is inconsistent with the general rules on which that 

 fact has been now shown to depend. Notwithstanding these erroneous views, the 

 "essay upon single vision with two eyes" contains many valuable experiments and 

 remarks, the truth of which are independent of the theory they were intended to 

 illustrate. 



