MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 373 



obviously dissimilar pictures on the two retinae when a single object is viewed, while 

 the optic axes converge, must therefore be regarded as a new fact in the theory of 

 vision. 



It being thus established that the mind perceives an object of three dimensions by 

 means of the two dissimilar pictures projected by it on the two retinae, the following 

 question occurs : What would be the visual effect of simultaneously presenting to each 

 eye, instead of the object itself, its projection on a plane surface as it appears to that 

 eye ? To pursue this inquiry it is necessary that means should be contrived to make 

 the two pictures, which must necessarily occupy different places, fall on similar parts 

 of both retinae. Under the ordinary circumstances of vision the object is seen at 

 the concourse of the optic axes, and its images consequently are projected on similar 

 parts of the two retinae ; but it is also evident that two exactly similar objects maybe 

 made to fall on similar parts of the two retinae, if they are placed one in the direction 

 of each optic axis, at equal distances before or beyond their intersection. 



Fig. 2. represents the usual situation of an object at the intersection of the optic 

 axes. In fig. 3. the similar objects are placed in the direction of the optic axes before 

 their intersection, and in fig. 4. beyond it. In all these three cases the mind per- 

 ceives but a single object, and refers it to the place where the optic axes meet. It 

 will be observed, that when the eyes converge beyond the objects, as in fig. 3., the 

 right hand object is seen by the right eye, and the left hand object by the left eye ; 

 while when the axes converge nearer than the objects, the right hand object is seen 

 by the left eye, and conversely. As both of these modes of vision are forced and un- 

 natural, eyes unaccustomed to such experiments require some artificial assistance. 

 If the eyes are to converge beyond the objects, this may be afforded by a pair of 

 tubes (fig. 5.) capable of being inclined towards each other at various angles, so as 

 to correspond with the different convergences of the optic axes. If the eyes are to 

 converge at a nearer distance than that at which the objects are placed, a box (fig. 6.) 

 may be conveniently employed ; the objects a a! are placed distant from each other, 

 on a stand capable of being moved nearer the eyes if required, and the optic axes being 

 directed towards them will cross at c, the aperture h V allowing the visual rays from 

 the right hand object to reach the left eye, and those from the left hand object to 

 fall on the right eye ; the coincidence of the images may be facilitated by placing the 

 point of a needle at the point of intersection of the optic axes c, and fixing the eyes 

 upon it. In both these instruments (figs. 5. and 6.) the lateral images are hidden 

 from view, and much less difficulty occurs in making the images unite than when 

 the naked eyes are employed. 



Now if, instead of placing two exactly similar objects to be viewed by the eyes in 

 either of the modes above described, the two perspective projections of the same solid 

 object be so disposed, the mind will still perceive the object to be single, but instead 

 of a representation on a plane surface, as each drawing appears to be when separately 



