10-4 On the Optical Illusion of the 



luminated by both, the elevation will again sink into a cavity, 

 as in Fig. 16. 



If the object A, in place of being a cavity, is actually the 

 raised segment of a solid sphere, the same phenomena will be 

 observed, the inverting eye-piece converting it into a cavity. 

 These two experiments may be made most successfully with a 

 seal, and an impression taken from it. 



It cannot therefore be doubted, that the optical illusion of 

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

 into a cameo, by an inverting eye-piece, 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. The greater our knowledge is, of this subject, 

 the more readily does the illusion seize upon us ; while, 

 if we are but imperfectly acquainted with the effects of 

 light and shadow, the more difficult is it to be deceived. If 

 the hollow is not polished, but ground, and the surface round 

 and of uniform colour and smoothness, almost every person, 

 whether young or old, will be subject to the illusion ; but if 

 the object is the raised impression of a seal upon wax, we 

 have often found that, when viewed with the eye-piece, it still 

 seemed raised to the three youngest of six persons, while the 

 three eldest were subject to the deception. By such trifling, 

 and often unappreciable circumstances, is our judgment af- 

 fected, that the same person at one moment sees the convexi- 

 ty raised, and at another time depressed, though viewed as 

 nearly as possible under the 'same circumstances. This re- 

 markable effect no doubt arises from the introduction of some 

 casual reflected lights, which the slightest change of position 

 will produce. 



Having thus seen how our judgment concerning elevations 

 and depressions is affected by our degree of knowledge of the 

 effects of light and shade, and by unappreciable causes, we 

 shall proceed to consider how our judgment is again deceived 

 by the introduction of new circumstances. 



Let the depression A, illuminated by one candle, as in 

 Fig. 14, be converted into an elevation as in Fig. 15, by the 

 application of an inverting eye-piece ; then, if another candle 

 C, Fig. 16, is introduced so as to illuminate the depression 



