282 



The Colour Sensations in Terms of Luminosity. 



" The Colour Sensations in Terms of Luminosity." By Captain 

 W. de W. Abney, C.B., D.C.L., F.B.S. Beceived February 23, 

 —Bead June 15, 1899. 



(Abstract.) 



This paper deals with a determination of the colour sensations 

 based on the Young theory by means of measures of the luminosity 

 of the three different colour components in a mixed light which 

 matches white. At the red end of the spectrum there is only one 

 colour extending to near C, and there is no mixture of other colours 

 which will match it, however selected. At the violet end of the spec- 

 trum, from the extreme violet to near G, the same homogeneity of 

 light exists, but it is apparently due to the stimulation of two sensa- 

 tions, a red and a blue sensation, the latter never being felt unmixed 

 with any other. Having ascertained this, it remained to find that 

 place in the spectrum where the blue sensation was to be found 

 unmixed with any other sensation except white. By trial it was found 

 that close to the blue lithium line this was the case, and that a 

 mixture of this colour and pure red sensation gave the violet 

 <of the spectrum when the latter was mixed with a certain quantity 

 of white. The red and the blue sensation being located, it remained 

 to find the green sensation. The complementary colour to the 

 red in the spectrum gave a position in which the green and blue 

 sensations were present in the right proportions to make white, and 

 a point nearer the red gave a point in which the red and blue sensa- 

 tions were present in such proportions as found in white, but 

 there was an excess of green sensation. By preliminary trials this 

 point was found. The position in the spectrum of the yellow 

 colour complementary to the violet was also found.' The colour of 

 bichromate of potash was matched by using a pure red and the 

 last-named green. To make the match, white had to be added to the 

 bichromate colour. A certain small percentage of white was found 

 to exist in the light transmitted through a bichromate solution with 

 which the match was made, and this percentage and the added white 

 being deducted from the green used, gave the luminosity of the pure 

 green sensation existing in the spectrum colour which matched the 

 bichromate. Knowing the percentage composition in luminosity of 

 the two sensations at this point, the luminosity of the three sensations 

 in white was determined by matching the bichromate colour with the 

 yellow (complementary to the violet) and the pure red colour sensa- 

 tion. From this equation and from the sensation equation of the bi- 

 chromate colour already found, the composition of the yellow was 

 determined. By matching white with a mixture of the yellow and the 



