Dr Brewster on a New Analysis of Solar Light. 205 



duced by the combustion of alcohol and water, or of an alco- 

 holic solution of salt. At a still greater thickness of the glass 

 we produce a greenish-white band, which, by changing the 

 glass for a different blue, becomes a reddish-white band. If 

 we now mix a solution of sulphate of copper, which acts upon 

 the rays on the red side of the yellow space, with diluted red 

 ink, which acts on the rays on the blue side of the same space, 

 we shall reduce the rays in the yellow space to nearly white 

 light, with a slight tinge of green, when there is too much 

 sulphate of' copper, and a slight tinge of red when there is too 

 much red ink. This insulation of white light may be pretty 

 well effected by some of the smalt blue glasses acting alone ; 

 and in some cases the purity of the light may be increased by 

 a solution of sulphate of copper and iron, or even by a green 

 glass. The white light thus exhibited may be rendered yel- 

 low by means of a yellow transparent wafer, which absorbs 

 some of its blue rays, and green by a green transparent wafer, 

 which absorbs some of its red rays. 



From these experiments it follows, that white light, compos- 

 ed of red, yellow, and blue rays, exist in the most luminous part 

 of the spectrum, and may be insulated by absorbing the excess 

 of yellow light, and of any of the other colours above what is 

 necessary to compose white light. In applying a highly dis- 

 persing prism, it was a singular and peculiarly interesting sight 

 to witness, for the first time, a beam of white light, consisting 

 of red, yellow, and blue rays of equal refrangibility, and inca- 

 pable of being analyzed by prismatic refraction. 



The preceding observations contain only a few out of a 

 great number of experiments which I have made on the ab- 

 sorptive action of natural and artificial crystals, and of various 

 fluids and uncrystallized solids which possess either a natural 

 or an artificial colour. I made a few experiments in the course 

 of this winter with the coloured juices of several hot-house 

 plants, which Mr Forbes was so good as to prepare for me ; 

 and I expected, in the course of the summer, to obtain by this 

 means, a more striking insulation of some of the simple colours, 

 than I had effected by the substances within my reach. Im- 

 patient, however, of so long a delay, I thought of supplying, 

 to a certain degree, the place of these absorbing fluids by a 

 subsidiary principle of analysis, which, in its practical applica- 



