452 Sir David Brewster on the Law of Visible Position 



indetermination of the judgement, arising from our more perfect 

 means of judging being absent*." 



Now, what Mr. Wheatstone calls the real cause of the illu- 

 sion is no cause at all, — it is merely a previous state of the 

 mind which is favourable to the operation of the real cause. 

 Two eyes, like two witnesses, must always bear a better testi- 

 mony to truth than one; and, in the present case, the want of 

 the convergency of the optic axes to estimate the distance of 

 the highest and lowest points of the cameo and the intaglio, 

 undoubtedly favours the illusion, and allows the real cause to 

 influence the judgement; but even here this admission has its 

 limits, for in very shallow cameos and intaglios the illusion 

 takes place with both eyesf. 



Without repeating in this place the various facts respecting 

 mother-of-pearl and other phaenomena 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'. M Lady Georgiana," says the Rev. 

 Mr. Wolff, " observed 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 J." 



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 Professor Necker 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 Edin- jpj„ jg 

 burgh Philosophical Journal §. Mr. Necker de- 

 scribes 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 spectator, and the solid angle X the furthest 



* Philosophical Transactions, 1838, p. 384. 



f 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 en- 

 croaches 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 in- 

 taglio seen very obliquely will not rise into a cameo, because the shadow 

 on the plane surface is wanting. 



t Journal of the Rev. Joseph Wolff, 1839, p. 189. 



§ Vol. i. p. 334. 



