CONTRIBUTIONS TO SCIENCE. 467 



But white light is not necessarily of so complex a nature 

 as sunlight ; thus it may consist of a mixture of red, green, 

 and blue lights simply, or of yellow and blue, or greenish- 

 yellow and violet, or of other mixtures of two or more kinds 

 of light, each in itself homogeneous. When this is the case, 

 the constitution of the light is revealed when it is allowed 

 to pass through a prism, for it is then separated into its 

 simple constituents which, on emerging from the prism, 

 pursue different paths. Similarly, various kinds of homo- 

 geneous light may be matched by means of a mixture of 

 lights of other colours appropriately chosen. Thus orange 

 light of a particular hue may be the homogeneous light 

 of the spectrum, or it may be a mixture of red and yellow 

 or of red and green lights, which is chromatically identical 

 with the homogeneous orange, but optically different, inas- 

 much as the mixture can be resolved into its constituents 

 by means of a prism, while the homogeneous orange light 

 may be passed through any number of prisms and yet retain 

 its perfect homogeneity. 



These results were accounted for by Thomas Young (1801), 

 on the suppositiqn that there are three separate sensations 

 which are excited to different degrees the proportion in 

 which each is excited depending on the nature of the light. 

 He was of opinion that these three sensations were red, 

 green, and violet, and that all other hues were compound 

 colours, though they might correspond to a simple kind of 

 light. " The quality' of any colour depends, according to this 

 theory, on the ratio of the intensities of the three sensations 

 which it excites, and its brightness depends on the sum of 

 these three intensities." 



Young's colour diagram was a triangle, at the angular 

 points of which he placed the three colours corresponding, 

 according to his theory, with the primary sensations ; along 

 the sides of the triangle were placed the colours formed by 

 mixing these two and two together, while within the triangle 

 were to be found the colours resulting from mixtures of all 

 three in different proportions. The position of each colour 

 was determined by finding the centre of gravity, of two or 



