PROCEEDINGS OP THE SOCIETY. . 491 



ception of the projection of the solid from which they are taken. But 

 if the figure, instead of being solid, was hollow, and was placed 

 before the eyes so as to show its interior, then the right eye would 

 see more of the left side, and the left eye would see more of the right 

 side fas in figs. 81 e and 81 d) ; and when these two pictures are put 

 into the Stereoscope, the centre appears to recede in an unmistakable 

 manner. I have here a stereoscopic slide showing these two pairs of 

 pictures at the same time ; and if you look at it in the Stereoscope, 

 the " conversion of relief " produced by the " crossing " of the right 

 and the left hand pictures of the pyramid, will be at once apparent. 



But in addition to these impressions of solid form, another very 

 curious fact now comes out. The four small squares are of exactly 

 the same size in the pictures ; and yet as you look at them in the 

 Stereoscope, you will all say that the square at the end of the hollow 

 pyramid seems larger than the other. The apparent excess differs 



Fig. 81. 



in different persons ; for some see the receding pyramid as if 

 much deeper than others, and describe it as like a tunnel ; and to 

 them the small square looks very much larger. This is another 

 case of mental suggestion, and one which no optical diagrams can 

 explain, because it is clear that the retinal pictures must be of exactly 

 the same size, however different they may seem visually. Sir Charles 

 Wheatstone in his second Memoir (Phil. Trans., 1852) clearly 

 proved, by experiments made with his improved reflecting Stereoscope, 

 that our conception of the size of an object pictured on the retina 

 ordinarily depends on our appreciation of its distance ; and that 

 this again (in the case of a near object), depends upon the convergence 

 of our optic axes. If we have an object of known size, and we bring 

 it nearer to the eye, it does not seem to be a larger object : because 

 we know that though it subtends a wider visual angle, making the 

 retinal picture larger, its distance from us has diminished. And 

 he showed that by making the optic axes converge, and so suggesting 

 to the mind that the object was approaching, though it was not brought 



