98 LETTERS ON NATURAL MAGIC. 



LETTER V. 



Miscellaneous optical illusions Conversion of cameos into 

 intaglios, or elevations into depressions, and the reverse 

 Explanation of this class of deceptions Singular 

 effects of illumination with light of one simple colour 

 Lamps for producing homogeneous yellow light Methods 

 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 im- 

 pressed, and the study of which is pregnant with 

 instruction and amusement. 



One of the most curious of these is that false 

 perception in vision by which we conceive de- 

 pressions to be elevations, and elevations depres- 

 sions, or by which intaglios are converted into 

 cameos, and cameos into intaglios. This curious 

 fact seems to have been first observed at one of 

 the early meetings of the Royal Society of Lon- 

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



