Dr BREWSTER on a New Analysis of Solar Light. 133 



the red space. That the blue extends over the whole red space 

 is proved by the same arguments which we used for yellow light ; 

 for when the red space is made of an orange tinge by the absorp- 

 tive action of certain yellow, orange, and green media, this change 

 can be effected only by the absorption of blue light. 



Having thus proved that red, yellow, and blue light exist in 

 almost every part of the spectrum, I shall proceed to corroborate 

 these views by shewing that white light may be actually in- 

 sulated in different parts of it. 



When we look at the spectrum through a particular blue glass 

 of a certain thickness, we insulate the yellow space, the colour 

 of which is a rich gamboge-yellow. By increasing the thickness 

 of the glass, this compound yellow acquires the pale straw-yel- 

 low colour of the yellow monochromatic flame produced by the 

 combustion of alcohol and water, or of an alcoholic 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-ivhite 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 yellowish-green juices ob- 

 tained from the green leaves of plants, and likewise some of the 

 lilac juices, such as that of the black currant, will be found use- 

 ful in rendering whiter this luminous band when produced by 

 blue glass. The white light thus exhibited may be rendered 



VOL. XII. PART I. S 



