248 Prof. Wheatstone on the Physiology of Vision. 



louring may properly be employed to heighten the effects. Care- 

 ful attention would enable an artist to draw and paint the two 

 component pictures, so as to present to the mind of the observer, 

 in the resultant perception, perfect identity with the object re- 

 presented. Flowers, crystals, busts, vases, instruments of various 

 kinds, &c, might thus be represented so as not to be distin- 

 guished by sight from the real objects themselves. 



It is worthy of remark, that the process by which we thus be- 

 come acquainted with the real forms of solid objects, is precisely 

 that which is employed in descriptive geometry, 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 assigning its projections 

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

 which are not parallel to each other. In the problems of de- 

 scriptive 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 be- 

 fore 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. 



§5. 



A very singular effect is produced when the drawing origi- 

 nally 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 

 dimensions, 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 relation between the 

 proper figure and this, which I shall call its converse figure. 

 Those points which appear nearest the observer in the proper 

 figure seem the most remote from him in the converse figure, and 

 vice versa, so that the figure is, as it were, inverted j 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, oc- 



