Prof. Wheatstone on the Physiology of Vision. 249 



casion a cube to be perceived; wben changed in the manner 

 described, represent the frustum of a square pyramid with its 

 base remote from the eye : the cause of this is easy to understand. 

 This conversion of relief may be shown by all the pairs of 

 drawings from fig. 10 to 19. In the case of simple figures like 

 these the converse figure is as readily apprehended as the origi- 

 nal one, because it is generally a figure of as frequent occurrence ; 

 but in the case of a more complicated figure, an architectural 

 design, for instance, the mind, unaccustomed to perceive its con- 

 verse, because it never occurs in nature, can find no meaning in it. 



§6- 



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 draw- 

 ings, one presented to each eye, and of two real objects so pre- 

 sented 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 bead- 

 ing about one tenth of an inch in thickness. 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 follow- 

 ing effects 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 retina? 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 effect of relief was destroyed, and the com- 

 pound appearance was that of an outline representation on a 

 plane surface. 3rdly. When the cubes were so placed that the 

 Deflected image of one projected on the left retina the same pic- 

 ture as in the first case was projected on the right retina, and 

 conversely, the converse figure in relief appeared. 



§7. 



If a symmetrical object, that is one whose right and left sides 

 Bre 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 <li>tant from the two eyes, its two monocular projections 

 arc, it is easy to see, inverted fac-similes of each other. Thus 



