CONVERSION OF CAMEOS INTO INTAGLIOS. 173 



LETTER V. 



Miscellaneous optical illusions Conversions of cameos into intaglios 

 or elevations into depressions, and the reverse. Explanation of 

 this c/ass of deceptions Singular effects of illumination with 

 light of one simple colour Lamps for producing homogeneous 

 yellow light Meth^s of increasing the effect of this exhibition 

 Method of reading the inscription of coins in the dark Art of 

 deciphering the effaced inscription of coins Explanation of these 

 singular effects Apparent motion of the eyes in portraits 

 Remarkable examples of this Apparent motion of the features 

 of a portrait, when the eyes are made to move Remarkable 

 experiment of breathing light and darkness. 



IN the preceding letter I have given an account of the 

 most important instruments of Natural Magic which 

 depend on optical principles ; but there still remain 

 several miscellaneous phenomena on which the stamp of 

 the marvellous is deeply impressed, and the study of 

 which is pregnant with instruction and amusement. 



One of the most curious of these is that false percep- 

 tion in vision by which we conceive depressions to be 

 elevations and elevations depressions, or by which in- 

 taglios are converted into cameos, and cameos into in- 

 taglios. This curious fact seems to have been first 

 observed at one of the early meetings of the Royal Society 

 of London, when one of the members, in looking at a 

 guinea through a compound microscope of new construc- 

 tion, was surprised to see the head upon the coin de- 

 pressed, while other members could only see it embossed 

 as it really was. 



While using telescopes and compound microscopes, 

 Dr. Gmelin of Wurtemburg observed the same fact. The 



