HOMOGENEOUS YELLOW LIGHT. 181 



proportions all the various hues of nature may be pro- 

 duced. These three colours, when mixed in the propor- 

 tion in which they occur in the sun's rays, compose a 

 purely white light ; but if any body on which this white 

 light falls shall absorb, or stop, or detain within its sub- 

 stance any part of any one or more of these simple colours, 

 it will appear to the eye of that colour which arises from 

 the mixture of all the rays which it does not absorb, or of 

 that colour which white light would have if deprived of 

 the colours which are absorbed. Scarlet cloth, for example, 

 absorbs most of the blue rays and many of the yellow, and 

 hence appears red. Yellow cloth absorbs most of the blue 

 and many of the red rays, and therefore appears yellow, 

 and blue cloth absorbs most of the yellow and red rays. 

 If we were to illuminate the scarlet cloth with pure and 

 unmixed yellow light, it would appear yellow, because the 

 scarlet cloth does not absorb all the yellow rays, but 

 reflects some of them ; and if we illuminate blue cloth 

 with yellow light, it will appear nearly black, because it 

 absorbs all the yellow light, and reflects almost none of it. 

 But whatever be the nature and colour of the bodies on 

 which the yellow light falls, the light which it reflects 

 must be yellow, for no other light falls upon them, and 

 those which are not capable of reflecting yellow light must 

 appear absolutely black, however brilliant be their colour 

 in the light of day. 



As the methods now discovered of producing yellow 

 light in abundance were not known to the ancient con- 

 jurers, nor even to those of later times, they have never 

 availed themselves of this valuable resource. It has been 

 long known that salt thrown into the wick of a flame pro- 

 duces yellow light, but this light is mixed with blue and 

 green rays, and is, besides, so small in quantity that it 

 illuminates objects only that are in the immediate vicinity 

 of the flame. A method which I have found capable of 



