180 THE ABSORPTION OF LIGHT. [SECT. xix. 



seven colours is viewed through a thin plate of this glass, they 

 are all visible ; and, when the plate is very thick, every colour 

 is absorbed between the extreme red and the extreme violet, 

 the interval being perfectly black : but, if the spectrum be 

 viewed through a certain thickness of the glass intermediate 

 between the two, it will be found that the middle of the red 

 space, the whole of the orange, a great part of the green, a 

 considerable part of the blue, a little of the indigo, and a very 

 little of the violet, vanish, being absorbed by the blue glass ; 

 and that the yellow rays occupy a larger space, covering part 

 of that formerly occupied by the orange on one side, and by 

 the green on the other. So that the blue glass absorbs the 

 red light, which when mixed with the yellow constitutes 

 orange ; and also absorbs the blue light, which when mixed 

 with the yellow forms the part of the green space next to the 

 yellow. Hence, by absorption, green light is decomposed 

 into yellow and blue, and orange light into yellow and red. 

 Consequently the orange and green rays, though incapable of 

 decomposition by refraction, can be resolved by absorption, 

 and actually consist of two different colours possessing the 

 same degree of refrangibility. Difference of colour, therefore, 

 is not a test of difference of refrangibility, and the conclusion 

 deduced by Newton is no longer admissible as a general 

 truth. By this analysis of the spectrum, not only with blue 

 glass, but with a variety of coloured media, Sir David 

 Brewster, so justly celebrated for his optical discoveries, has 

 proved that the solar spectrum consists of three primary co- 

 lours, red, yellow, and blue, each of which exists throughout 

 its whole extent, but with different degrees of intensity in 

 different parts ; and that the superposition of these three 

 produces all the seven hues according as each primary colour 

 is an excess or defect. Since a certain portion of red, yellow, 

 and blue rays constitute white light, the colour of any point 

 of the spectrum may be considered as consisting of the 

 predominating colour at that point mixed with white light. 

 Consequently, by absorbing the excess of any colour at any 



