230 



Capt. W. de W. Abney. 



[Feb. 19, 



Here we have the colour Z in terms of a single ray, and of white 

 light. 



This same holds good when in (ii) 7 is smaller than and /3; but 

 it does not do so should it happen that ft is the smallest, for there is 

 no part of the spectrum which contains simple colours giving the 

 same sensation to the eye as mixtures of red and blue. There is, 

 however, a very simple way in which the registration of such a colour 

 (which it must be remarked must be of a purple tone) can be effected. 

 It can be fixed by its complementary. To do this we must add to (ii) 

 a certain amount of R and V, which will make the whole white. 

 Thus, suppose in (iii) to be larger than 7 and 7 than |3, then we 

 must add 06G + 0cV, and we have 



bnt 



Now, between V and in the spectrum there is some single color 

 which gives the sensation of the mixture of G and V. Let it be X* 

 with luminosity x', together with white, whose luminosity is p, which 

 equals 



and (7 + 0) each equal a. ; .'. n = aw 



Z= (*w- f i')W-x'X'; 



which again is the colour expressed in terms of white light less tl 

 complementary colour. We have thus arrived at the very simp 

 deduction that the hue and luminosity of any colour, however cot 

 pounded, may be registered by a reference to white light and a single 

 ray of the spectrum. 



In practice this dominant ray is very easy to find. Suppose we 

 wish to determine numerically the colour of a signal-green glass in 

 th electric light ; we should proceed as follows : 



The colour-patch apparatus described in the Appendix to the 

 Bakerian Lecture " On Colour Photometry " (' Phil. Trans.,' 1886, 

 Abney and Festing) is employed, and the coloured glass is placed 

 between the silvered mirror, which reflects the beam already reflected 

 from the first surface of the first prism of the spectrum apparatus, 

 and the screen, and a square image of that surface of the prism, 

 showing the tint of the glass, is formed on the screen by means of 

 the lens. Touching this image is a square patch of white light, 

 formed by the re-combination of the spectrum by means of another 

 lens. An opaque slide, containing an adjustable slit, is moved across 

 the spectrum in the manner described in the paper referred to, imtil 



