Conversion of Cameos into Intaglios, fyc 103 



any plain, afford an exception from that rule of vision, and do 

 not appear depressed when viewed after the method above 

 mentioned.'" 



In the year 1780, this subject occupied the attention of 

 David Rittenhouse, president of the American Philosophical 

 Society, who gave a correct explanation of the illusion, by 

 referring it to the inversion of the shadow by the eye-tube. 

 He employed in his observations an eye-piece, having two 

 lenses placed at a distance greater than the sum of their focal 

 distances ; and by throwing a reflected light on the cavities 

 observed, in a direction opposite to that of the light, which 

 came from his window, he was able to see them raised into 

 elevations, by looking through a tube without any lenses. Mr 

 Rittenhouse also observed, that, by putting his finger into the 

 cavity, the illusion ceased to take place. 



Having thus given a brief detail of the experiments of 

 Gmelin and Rittenhouse, we shall proceed to explain more mi- 

 nutely the principles on which this illusion depends. 



It will afterwards be seen, that inverting telescopes and mi- 

 croscopes are not necessary to the production of this illusion ; 

 but it may be best seen by viewing with the eye-piece of an ach- 

 romatic telescope the engraving upon a seal, when illuminated 

 either by a candle or the window of an apartment. This eye- 

 piece inverts the objects to which it is applied like the compound 

 microscope, and the excavations or depressions of the seal are 

 immediately raised up into elevations like a cameo, or a bas- 

 relief. The cause of this illusion will be understood from 

 Plate I. Fig. 14, where A represents a spherical cavity illu- 

 minated by a candle C. The shadow of the cavity will of 

 course be on the left side S, and, therefore, if we view it 

 through an inverting eye-piece or microscope, the cavity will 

 be seen as at A, Fig. 15, with its shadow on the right hand S 

 of the cavity. As the candle C remains where it was, the 

 observer instantly concludes, that what was formerly a cavity, 

 must now be a spherical elevation or segment of a sphere, as 

 nothing but a raised body could have its shadow on the right 

 hand S. If a second candle is now placed on the right hand 

 ;sidc of A, so that it is between two candle*, anil is equally il- 



