378 MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 



The same image is depicted on the retina by an object of three dimensions as by 

 its projection on a plane surface, provided the point of sight remain in both cases the 

 same. There should be, therefore, no difference in the binocular appearance of two 

 drawings, one presented to each eye, and of two real objects so presented to the two 

 eyes that their projections on the retina shall be the same as those arising from the 

 drawings. The following experiments will prove the justness of this inference. 



I procured several pairs of skeleton figures, i. e. outline figures of three dimensions, 

 formed either of iron wire or of ebony beading about one tenth of an inch in thick- 

 ness. The pair I most frequently employed consisted of two cubes, whose sides were 

 three inches in length. When I placed these skeleton figures on stands before the 

 two mirrors of the stereoscope, the following eflfects were produced, according as their 

 relative positions were changed. 1st. When they were so placed that the pictures 

 which their reflected images projected on the two retinae were precisely the same as 

 those which would have been projected by a cube placed at the concourse of the 

 optic axes, a cube in relief appeared before the eyes. 2ndly. When they were so 

 placed that their reflected images projected exactly similar pictures on the two retinae, 

 all eflfect of relief was destroyed, and the compound appearance was that of an out- 

 line representation on a plane surface. 3rdly. When the cubes were so placed that 

 the reflected image of one projected on the left retina the same picture as in the first 

 case was projected on the right retina, and conversely, the converse figure in relief 

 appeared. 



If a symmetrical object, that is one whose right and left sides are exactly similar to 

 each other but inverted, be placed so that any point in the plane which divides it into 

 these two halves is equally distant from the two eyes, its two monocular projections 

 are, it is easy to see, inverted fac-similes of each other. Thus fig. 15, a and b are sym- 

 metrical monocular projections of the frustum of a four-sided pyramid, and figs. 13. 

 14. 16. are corresponding projections of other symmetrical objects. This being kept 

 in view, I will describe an experiment which, had it been casually observed previous 

 to the knowledge of the principles developed in this paper, would have appeared an 

 inexplicable optical illusion. 



M and M' (fig. 21.) are two mirrors, inclined so that their faces form an angle of 

 90° with each other. Between them in the bisecting plane is placed a plane outline 

 figure, such as fig. 15 a, made of card all parts but the lines being cut away, or of 

 wire. A reflected image of this outline, placed at A, will appear behind each mirror 

 at B and B', and one of these images will be the inversion of the other. If the eyes 

 be made to converge at C, it is obvious that these two reflected images will fall on 

 corresponding parts of the two retinae, and a figure of three dimensions will be per- 

 ceived; if the outline placed in the bisecting plane be reversed, the converse skeleton 



