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 impressed, 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 depressions to be elevations, and elevations depressions, 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 London, when one of the members, in looking at a guinea through a compound microscope of new construction, was surprised to see the head upon the coin depressed, 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 protuberant parts of objects appeared to him depressed, and the depressed parts protuberant: but what perplexed him extremely, this illusion took place at some times and not at others, in some experiments and not in others, and appeared to some eyes and not to others.