Discovery of his Theory of Colours. 125 



produced by combining the whole of the green and violet 

 rays is not the blue of the spectrum ; for four parts of green 

 and one of violet make a blue differing very little from green; 

 while the blue of the spectrum appears to contain as much violet 

 as green ; and it is for this reason that red and blue usually 

 make a purple, deriving its hue from the predominance of 

 violet. 



" It would be possible to exhibit at once to the eye the com- 

 binations of any three colours in all imaginable varieties. Two 

 of them might be laid down on a revolving surface in the form 

 of triangles placed in opposite directions, and the third on 

 projections perpendicular to the surface, which, while the eye 

 remained at rest in any one point obliquely situated, would 

 exhibit more or less of their painted sides as they passed 

 through their different angular positions : and the only further 

 alteration that could be produced in any of the tints would be 

 derived from the different degrees of light only. The same 

 effect may also be exhibited by mixing the colours in different 

 proportions by means of the pencil, beginning from three equi- 

 distant points as the centres of the respective colours." 



On certain portions of the above extracts, which I have ita- 

 licized, I will venture a few observations. In the first passage 

 thus indicated, Young says : — " The spectrum formed in this 

 manner consists of four colours only, red, green, blue, and violet.'''' 

 Young here replacesWollaston's " yellowish green " by "green*" 

 and further on he adds, u The colours differ scarcely at all in 

 quality within their respective limits ; but they vary in bright- 

 ness, the greatest intensity of light being in that part of the 

 green which is nearest to the red," thus confirming our view 

 that both Young and Wollaston were of the opinion that, when 

 a narrow bright crevice is observed through a prism, the 

 spectrum so viewed consists of only four colours, red, green, 

 blue, and violet, " differing scarcely at all in quality within 

 their respective limits ;" which limits they supposed naturally 

 existed in the dark spaces which, as they imagined, bounded 

 these elementary colours. Young, however, somew T hat modifies 

 this opinion in the next sentence, when he says, " A narrow 

 line of yellow is generally visible at the limit of the red and 

 green ; but its breadth scarcely exceeds that of the aperture by 

 which the light is admitted, and Dr. Wollaston attributes it to 

 the mixture of the red with the green light." It would indeed 

 appear from the last portion of this sentence that Young ob- 

 tained directly from Wollaston one of the main facts on which 

 his theory was founded, namely that yellow can be reproduced 

 by the mixture of red and green lights. But Wollaston, in 

 his paper of 1802, from which we have cited, makes no such 



