MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 377 



It is worthy of remark, that the process by which we thus become acquainted with 

 the real forms of solid objects, is precisely that which is employed in descriptive geo- 

 metry, an important science we owe to the genius of Monge, but which is little 

 studied or known in this country. In this science, the position of a point, a right line 

 or a curve, and consequently of any figure whatever, is completely determined by as- 

 signing its projections on two fixed planes, the situations of which are known, and 

 which are not parallel to each other. In the problems of descriptive geometry the 

 two referent planes are generally assumed to be at right angles to each other, but in 

 binocular vision the inclination of these planes is less according as the angle made 

 at the concourse of the optic axes is less ; thus the same solid object is represented 

 to the mind by different pairs of monocular pictures, according as they are placed at 

 a different distance before the eyes, and the perception of these differences (though 

 we seem to be unconscious of them) may assist in suggesting to the mind the distance 

 of the object. The more inclined to each other the referent planes are, with the 

 greater accuracy are the various points of the projections referred to their proper 

 places ; and it appears to be a useful provision that the real forms of those objects 

 which are nearest to us are thus more determinately apprehended than those which 

 are more distant. 



A very singular effect is produced when the drawing originally intended to be seen 

 by the right eye is placed at the left hand side of the stereoscope, and that designed 

 to be seen by the left eye is placed on its right hand side. A figure of three dimen- 

 sions, as bold in relief as before, is perceived, but it has a different form from that 

 which is seen when the drawings are in their proper places. There is a certain rela- 

 tion between the proper figure and this, which I shall call its converse figure. Those 

 points which are nearest the observer in the proper figure are tlie most remote from 

 him in the converse figure, and vice versd, so that the figure is, as it were, inverted ; 

 but it is not an exact inversion, for the near parts of the converse figure appear 

 smaller, and the remote parts larger than the same parts before the inversion. Hence 

 the drawings which, properly placed, occasion a cube to be perceived, when 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 appre- 

 hended as the original 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 converse, because 4t never occurs in nature, 

 can find no meaning in it. 



MDCCCXXXVIII, 3 c 



