886 OF SENSATION, AND THE ORGANS OF THE SENSES. 



888. The Stereoscope, in its original form, essentially consists of two plane 

 mirrors, inclined with their backs to one another at an angle of 90. If two 

 perspective drawings of any solid object, as seen at a given distance with the 

 two eyes respectively, such as those at A and B, Fig. 214, be so placed before 

 these mirrors, one before each, that their two images shall be made to fall upon 

 the corresponding parts of the two retinae, in the same manner as the two images 

 formed by the solid object itself would have done, the mind will perceive, not 

 a single representation of the object, nor a confused union of the two, but a 

 projecting or receding surface, the exact counterpart of that from which the 

 drawings were made. 1 The solid form is forcibly impressed on the mind, even 

 when outlines only are given, especially if these be delineations of simple geo- 



Fig. 214. 



metrical figures, easily suggested to the mind; and it may be easily shown that 

 the very same outlines will suggest different conceptions, according to the mode 

 in which they are placed. Thus, in Fig. 215, the upper pair of figures A, B, 

 when combined in the Stereoscope, convey the idea of a projecting truncated 

 pyramid, with the small square in the centre, and the four sides sloping equally 

 away from it; whilst the lower pair of figures, c, D, which are the same as the 

 upper, but transferred to the opposite sides, no less vividly bring before the mind 

 the visual conception of a receding pyramid, still with the small square in the 

 centre, and the four sides sloping equally towards it. Prof. Wheatstone further 

 shows, by means of the Stereoscope, that similar images, differing to a certain ex- 

 tent in magnitude, when presented to the corresponding parts of the two retinae, 

 give rise to the perception of a single object, intermediate in size between the 

 two monocular pictures. Were it not for this, objects would appear single, 



the entire merit of the idea that all our perception of solidity derived through the visual 

 sense is consequent upon the mental combination of the two dissimilar pictures upon the 

 two retinae and further that the whole merit of the realization of that idea by means of 

 the mirror-stereoscope, long before Sir D. Brewster's attention had been given to the sub- 

 ject at all belongs to Prof. Wheatstone. 



1 The most striking effect is produced by two photographic pictures, taken at the same 

 time by two cameras, so placed that their axes shall form the same angle with each other 

 as that which the axes of the two eyes would form when looking at the same object. This 

 adaptation, though the credit has been assumed by others, was originally devised by Prof. 

 Wheatstone. 



