Dr BREWSTER on a New Analysis of Solar Light. 131 



in the blue and indigo spaces, may be inferred from another ex- 

 periment. When we transmit the spectrum through a certain 

 thickness of a blue solution 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 yel- 

 low rays, which cause its apparent whiteness. In order to ap- 

 prehend the force of this argument, we must consider, that though 

 a dark red or a dark blue fluid appears considerably 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 re- 

 ference to red and blue light, which each of them freely transmits, 

 they may be regarded as perfectly transparent. Nothing is more 

 remarkable to those who first make the experiment, than the im- 

 perceptible diminution of intensity which a beam of homogeneous 

 red light experiences in passing 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 thick- 

 nesses of balsam of sulphur, balsam of Peru, pitch, or red mica, 

 yellow light can be seen directly at the line marked C of FRAUN- 



