CHAP. iv.J ACTION OF COLOURED BODIES. 35 



constructing a monochromatic lamp for microscopical purposes 

 to prevent the coloured fringes produced by the imperfectly cor- 

 rected lenses then used in the construction of high power object- 

 glasses. He had observed that burning paper, linen, cotton, 

 &c., gave flames in which the yellow rays predominated, and 

 this effect was most marked in the flame of alcohol diluted 

 with water. On examining this flame with the aid of a prism, 

 he found that the great bulk of the light was located in the 

 yellow, and though there were faint traces of green and blue, 

 there was not a trace of red. So far, there is nothing 

 particularly new. But he next proceeded to open up a new 

 field of observation altogether. In the course of his experiments 

 he examined the action of various coloured bodies on the different 

 parts of the spectrum. He found not only that different coloured 

 bodies cut out different rays from the spectrum, but that 

 this action of the same body varied with its thickness and 

 temperature. 



Shortly after this paper appeared, Mr. J. W. F. Herschel, 1 in 

 a letter to Sir David Brewster, described a similar series of 

 experiments which he had made. In one of these he projected 

 a spectrum obtained by a circular opening and a prism on a 

 sheet of white paper, and examined it through a glass which cut 

 out all but the red rays. He found that the light was perfectly 

 hornogeneous, and produced a well-defined circular image of the 

 aperture. 



Here then was an instance of light coloured red, not because 

 it was originally tuned to vibrate at that one rate only, for 

 the light was originally complete, and gave the continuous 

 spectrum, 



w a g Y ^ 



but because in its passage through the glass the violet, indigo, 



1 Edin. Phil. Trans, vol. ix. 1823, p. 445 et seq. 



D 2 



