366 SIR DAVID BREWSTER ON THE LAW OF VISIBLE POSITION 



for in very shallow cameos and intaglios the illusion takes place with both 

 eyes.* 



Without repeating in this place the various facts respecting mother-of-pearl 

 and other phenomena in which I observed the illusion when both eyes were used, 

 I shall content myself with quoting the following observation, made in Egypt by 

 Lady GEORGIANA WOLFF. " Lady GEORGIANA," says the Rev. Mr WOLFF, " ob- 

 served a curious optical deception in the sand about the middle of the day, when 

 the sun was strong ; all the foot-prints, and other marks that are indented in the 

 sand, had the appearance of being raised out of it ; and at those times there was such 

 a glare that it was unpleasant for the eye."f 



8. On the Change in the Apparent Position of the Drawings of Solid Bodies. 



Although this illusion may have been previously observed, yet I believe Pro- 

 fessor NECKEB of Geneva is the first person who has described and explained it. 

 He mentioned it to me in conversation in 1832 ; and afterwards sent me a notice 

 of it, which I published in the London and Edinburgh Philosophical Journal.:): 

 Mr NECKER describes the illusion in the following manner. " The rhomboid AX, 

 fig. 19, is drawn so that the solid angle A should be seen the nearest to the spec- 

 tator, and the solid angle X the farthest from him, and that the face ACBD 

 should be the foremost while the face XDC is behind. But in looking repeatedly 

 at the same figure, you will perceive that at times the apparent position of the 

 rhomboid is so changed that the solid angle X will appear the nearest, and the 

 solid angle A the farthest, and that the face ACDB will recede behind the face XDC, 

 which will come forward ; which effect gives to the whole solid a quite contrary 

 apparent inclination." Professor NECKER observed this change " as well with one 

 as with both eyes," and he considered it as owing " to an involuntary change in 

 the adjustment of the eye for obtaining distinct vision. And that whenever the 

 point of distinct vision on the retina was directed on the angle A, for instance, 

 this angle seen more distinctly than the others, was naturally supposed to be 

 nearer and foremost ; while the other angles seen indistinctly were supposed to be 

 farther away and behind. The reverse took place when the point of distinct vision 

 was brought to bear upon the angle X." Upon this explanation Mr WHEATSTONE 

 makes the following observations : " That this is not the true explanation is evi- 



* When the cameo and intaglio are viewed very obliquely, one of the causes of deception disappears. 

 In the case of a cameo appearing depressed, the depression disappears the instant that the shadow of 

 the cameo encroaches distinctly upon the plane surface from which it is raised, because an intaglio never 

 can, however obliquely viewed, throw a shadow upon the plane surface out of which it is excavated. For 

 the same reason, an intaglio seen very obliquely will not rise into a cameo, because the shadow on the 

 plane surface is wanting. 



t -Journal of the Eev. Joseph Wolff, 1839, p. 189. | Vol. i. p. 334. 



