244 Prof. Wheatstone on the Physiology of Vision. 



section 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 perceives 

 but a single object, and refers it to the place whei'e the optic 

 axes meet. It will be observed, that when the eyes converge 

 beyond the objects, as in tig. 3, the right-hand object is seen by 

 the right eye, and the left-hand object by the left eye j but 

 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 unnatural, 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 conver- 

 gences 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 bb' 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 coinci- 

 dence 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 dis- 

 posed, 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 viewed by that eye which is di- 

 rected towards it, the observer will perceive a figure of three 

 dimensions, the exact counterpart of the object from which the 

 drawings were made. To make this matter clear, I will mention 

 one or two of the most simple cases. 



If two vertical lines near each other, but at different distances 

 from the spectator, be regarded first with one eye and then with 

 the other, the lateral separation between them when referred to 

 the same plane will appear different ; if the left-hand line be 

 nearer to the eyes, the separation seen by the left eye will be less 

 than that seen by the right eye; fig. 7 will render this evident; 

 a a 1 are vertical sections of the two original lines, and b b' the 

 plane to which their projections are referred. If now the two 



