MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 883 



§11. 



The same indetermination of judgement which causes a drawing to be perceived 

 by the mind at different times as two different figures, frequently gives rise to a false 

 perception when objects in relief are regarded with a single eye. The apparent con- 

 version 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 attention, was first ob- 

 served 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 Philosophical Transactions for 1745 ; 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 thingvS 

 should so happen," says he, " I do not pretend to determine." 



Sir David Brewster accounts for the fallacy in the following manner-|- : — "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, over- 

 comes 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 could 

 not have taken place. The illusion, therefore, imder 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 persons 

 are under its influence, others are entirely insensible to it." 



These considerations do not fully explain the phenomenon, for they suppose that 

 the image must be inverted, and that the light must fall in a particular direction ; 

 but the conversion of relief will still take place when the object is viewed through an 



* Birch's History, vol. ii. p. 348. t Natural Magic, p. 100. 



