SECT. XIX. CONSTITUTION OF LIGHT. 161 



the yellow rays occupy a larger space, covering part of that for- 

 merly 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, there- 

 fore, 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 Avith blue glass but 

 with a variety of coloured media, Sir David Brewster, so justly 

 celebrated for his optical discoveries, is of opinion that the solar 

 spectrum consists of three primary colours, red, yellow, and blue, 

 each of which exists throughout its whole extent, but with dif- 

 ferent degrees of intensity in different parts ; and that the super- 

 position of these three produces all the seven hues according as 

 each primary colour is in excess or defect. That 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 con- 

 sisting of the predominating colour at that point mixed with 

 white light. Consequently, " by absorbing the excess of any 

 colour at any point of the spectrum above what is necessary to 

 form white light, such white light will appear at that point as 

 never mortal eye looked upon before this experiment, since it 

 possesses the remarkable property of remaining the same after 

 any number of refractions, and of being capable of decomposition 

 by absorption alone." This analysis of light has been called in 

 question by Professor Challis, of Cambridge, who does not admit 

 of any resolution by absorbing media different from that by the 

 prism, though he admits that a mixture of blue and yellow solar 

 light produces green. Professor Stokes, on the contrary, does not 

 allow that a mixture of blue and yellow solar light produces 

 green, although that mixture produces green when the light is 

 from other sources, for he found the gradation from sunlight to 

 pass from yellow through diluted yellow, white, diluted blue to 

 blue ; so he does not admit of Sir David Brewster's analysis of 



