2 LETTERS ON NATURAL MAGIC. 



COLOUR OF THE COLOUR OP THE 



ORIGINAL FIGURES. SPECTRAL FIGURES, 



Green, Reddish-violet, 



Blue, Orange-red. 



Indigo, Orange-yellow. 



Violet, Yellow. 



White, Black. 



Black, White. 



The two last of these experiments, viz., white 

 and black figures, may be satisfactorily made by 

 using a white medallion on a dark ground, and a 

 black profile figure. The spectrum of the former 

 will be found to be black, and that of the latter 

 white. 



These ocular spectra often show themselves 

 Avithout any effort on our part, and even without 

 our knowledge. In a highly painted room, illu- 

 minated by the sun, those parts of the furniture 

 on which the sun does not directly fall have 

 always the opposite or accidental colour. If the 

 sun shines through a chink in a red window-cur- 

 tain, its light will appear green^ varying as in the 

 above table, with the colour of the curtain ; and 

 if we look at the image of a candle, reflected 

 from the water in a blue finger-glass, it will ap- 

 pear yellow. Whenever, in short, the eye is 

 affected with one prevailing colour, it sees at the 

 same time the spectral or accidental colour, just 

 as when a musical string is vibrating, the ear 

 hears at the same time its fundamental and its 

 harmonic sounds. 



If the prevailing light is white and very strong, 

 the spectra which it produces are no longer black, 

 but of various colours in succession. If we look 

 at the sun, for example, when near the horizon, 

 or when reflected from glass or \yater so as to 



