Dr Brewster on a New Analysis of Solar Light. 203 



mit the spectrum through a certain thickness of a hlue solu- 

 tion of the ammonio-sulphate of copper, the blue and indigo 

 spaces appear to be much diluted with white light, that is, 

 the blue appears to be mixed with red and yellow. Now, if 

 this apparently diluted blue light is a pure homogeneous blue, 

 containing neither red nor yellow rays, it would suffer no more 

 diminution in passing through an additional thickness of the 

 ammonio-sulphate, than white light would do in passing 

 through the same thickness of pure crystal or pure water, that 

 is, it would suffer no perceptible change. But, in passing 

 through the copper solution, the blue becomes rapidly deeper 

 and less white, which can arise only from its absorbing the 

 red and yellow rays, which cause its apparent whiteness. In 

 order to apprehend the force of this argument, we must con- 

 sider, that though a dark red or a dark blue fluid appear 

 opaque, as they are in reference to white light, of which the one 

 absorbs all the rays but the red, and the other all but the blue, 

 yet, in reference to red and blue light, which each of them 

 freely transmits, they may be regarded as perfectly transpa- 

 rent. Nothina; is more remarkable to those who first make 

 the experiment, than the imperceptible diminution of intensity 

 which a beam of homogeneous red light experiences in pas- 

 sing through a great thickness of a red fluid, particularly when 

 the original red beam is produced by transmission through 

 the same red fluid. It is owing to this cause that the colour 

 of a wine-glass, full of port wine, is nearly as deep as that of 

 the wine in the thickest part of a wine decanter. 



That yellow light exists in every part of the red space, may 

 be proved by numerous experiments. By using a prism of 

 port wine of 90°, or by viewing the spectrum through certain 

 thicknesses of balsam of sulphur, balsam of Peru, pitch, or red 

 mica, yellow light can be seen directly at the line marked C 

 of Fraunhofcr, which is far within the red space ; and, by 

 the absorptive action of these four last substances, the whole 

 of the red space has a yellowish tint, arising from the absorp- 

 tion of blue light. The very same effect is produced, but in a 

 more striking manner, by transmitting the light of the red 

 space through certain yellow, orange, and green transparent 

 wafers, all of which absorb some blue light, and leave the whole 



