Prof. Wheatstone on the Physiology of Vision. 255 



objects in relief are regarded with a single eye. The apparent 

 conversion of a cameo into an intaglio, and of an intaglio into a 

 cameo, is a well-known instance of this fallacy in vision ; but 

 the fact does not appear to me to have been correctly explained, 

 nor the conditions under which it occurs to have been properly 

 stated. 



This curious illusion, which has been the subject of much at- 

 tention, was first observed at one of the early meetings of the 

 Royal Society*. Several of the members looking through a 

 compound microscope of a new construction at a guinea, some 

 of them imagined the image to be depressed, while others 

 thought it to be embossed, as it really was. Professor Gmelin, 

 of Wurtemburg, published a paper on the same subject in the 

 Phdosophical Transactions for 1 745 ; his experiments were made 

 with telescopes and compound microscopes which inverted the 

 images ; and he observed that the conversion of relief appeared 

 in some cases and not in others, at some times and not at others, 

 and to some eyes also and not to others. He endeavoured to 

 ascertain some of the conditions of the two appearances ; " but 

 why these things should so happen," says he, " I do not pretend 

 to determine." 



Sir David Brewster accounts for the fallacy in the following 

 manner f : — " A hollow seal being illuminated by a window or a 

 candle, its shaded side is of course on the same side with the 

 light. If we now invert the seal with one or more lenses, so 

 that it may look in the opposite direction, it will appear to the 

 eye with the shaded side furthest, from the window. But as we 

 know that the window is still on our left hand, and as every 

 body with its shaded side furthest from the light must necessarily 

 be convex or protuberant, we immediately believe that the hollow 

 seal is now a cameo or bas-relief. The proof which the eye thus 

 receives of the seal being raised, overcomes the evidence of its 

 being hollow, derived from our actual knowledge and from the 

 sense of touch. In this experiment the deception takes place 

 from our knowing the real direction of the light which falls on 

 the seal; for if the place of the window, with respect to the 

 seal, had been inverted as well as the seal itself, the illusion 

 Mold not have taken place. The illusion, therefore, under our 

 consideration is the result of an operation of our own minds, 

 whereby we judge of the forms of bodies by the knowledge we 

 have acquired of light and shadow. Hence the illusion depends 

 on the accuracy and extent of our knowledge on this subject; 

 and while some person* are under its influence, others are en- 

 tirely insensible to it." 



• Birch's History, vol. ii. |). 348. f Natural Magic, p. 100. 



