PROCEEDINGS OF THE SOCIETY. 489 



tures of the same kind in tlie Bourse, by which the impression of 

 solidity is so well given, that, though the paintings are quite flat, they 

 are generally taken by strangers for real relievos. I have a photo- 

 graph of a figure in such low relief, that, looking at it with both eyes 

 at a distance of only two feet, you could almost swear to its solidity ; 

 the suggestion of solidity given by its lights and shadows being so 

 vivid, as to overcome the corrective effect of the binocular perception 

 of its flatness. 



I dwell upon this point, because it underlies the whole inquiry 

 before us. I have here four large photographs of plaques repre- 

 senting the Tour Seasons, with the ornamentation and figures in high 

 relief. When you look at three of these with one eye, you will 

 scarcely be able to persuade yourselves that you are not seeing actual 

 relievos, so vividly do the figures stand out. But I have hung one of 

 them upside down ; and though you may not all see it as I do, I think 

 the impression upon most persons will be that the figures are hollowed 

 out, instead of raised. In each case the illusion depends mainly upon 

 the light ; and it is most complete when there is but one source of 

 light in the room, corresponding with the lights in the photograph. 

 The mental impression is entirely due to suggestion ; you know the 

 position of the light, and can tell in 

 which direction the shadows would Fig. 



fall ; and when the shadow is made to 

 fall as it would if the object were 

 hollow, then the mind interprets the 

 object as such. Another remarkable 

 instance of suggestion is afforded by 

 this figure of a rhomb (fig. 80), which, 

 as you look at it, may seem to change 

 from one position to another, some- 

 times appearing to stand upon its 

 narrow side, at other times to be lying 

 on its broad side. Sir D. Brewster 

 says that the perception changes from one to the other, as you feel 

 your mind changing ; but I believe that the perceptional and therefore 

 the mental change is the result of the wandering of the eye from the 

 point a to the point 6 ; for I have never failed to see one or the 

 other aspect, by making my eyes converge upon one or the other of 

 these two points, which then becomes the salient angle. This is a 

 case in which two different effects of projection may be produced by 

 the same visual impression ; a consideration much dwelt upon by 

 Sir Charles Wheatstone in his original memoir,* as proving that the 

 conception of solid form is visually suggested to the mind, not a mere 

 optical effect. 



I now come to the subject of Binocular Stereoscopic vision, which 

 was first elucidated in that memoir. Painters had long been aware 

 of the fact, that if you look at a near object with both eyes, you form 

 different pictures with your two eyes. How is it, then, that we are not 



* " On some remarkable and hitherto unobserved phenomena of Binocular 

 Vision." Phil. Trans., 1838, pp. 371-94. 



