COMMON THINGS COLOUR. 



Tesolved into two equally refrangible, one being yellow and the 

 other blue. 



29. In a word, the observations and experiments of Sir David 

 Brewster have led him to the conclusion that the prismatic 

 spectrum consists in reality of three spectra of nearly equal length, 

 each of uniform colour, superposed one upon another ; and that 

 the colours which the actual spectrum exhibit arise from the mix- 

 ture of the uniform colours of these three spectra superposed. 

 The colours of these three elementary spectra, according to Sir 

 David Brewster, are red, yellow, and blue. He shows that by 

 the combination of these three, not only all the colours exhibited 

 in the prismatic spectrum may be reproduced, but that their 

 combination also produces white light. He contends, therefore, 

 that the white light of the sun consists not of seven, but of three 

 constituent lights, red, yellow, and blue. 



This conclusion is established by showing that there is 

 another method by which light may be resolved into its com- 

 ponents, besides the method of refraction by prisms. In passing 

 through certain coloured media, it is admitted that a portion of 

 the light incident is intercepted at the surface upon which it is 

 incident, and in its passage through the medium a part only is 

 transmitted. 



Now, this property of colours is taken by Sir David Brewster 

 as another method, independently of refraction, of decomposing 

 colours. He assumes that such a medium resolves the light inci- 

 dent upon it into two parts ; first, the part which it transmits ; 

 and, secondly, the part which it intercepts. He concludes that 

 these two parts are complementary, that is to say, that each con- 

 tains what the other wants to make up white solar light ; or, more 

 generally, that the incident light, whatever be its nature, must be 

 -assumed to be a compound, consisting of the light transmitted and 

 the light intercepted. 



This being assumed, let a coloured medium, such as a plate 

 t)f blue glass, be held between the eye and the spectrum. Cer- 

 tain colours of the spectrum will be transmitted and others inter- 

 cepted. If the colours of the spectrum be simple and homogeneous 

 light, such as they are assumed to be in the Newtonian theory of 

 the decomposition of light, then the consequence would be that 

 the appearance of the spectrum seen through the coloured medium 

 would consist of dark and coloured spots; those simple lights 

 intercepted by the glass appearing dark, and those transmitted by 

 the glass having their proper colour. For if each colour of the 

 prism be, as is assumed in the chromatic theory, simple, 

 then the plate of glass can make no change in its colour by 

 transmission. 

 78 



