160 LORD RAYLEIGH ON THE COLOURS OF THIN PLATES. 



" (24) corresponds to a bright scarlet about one-third of the distance from 

 C to D ; (44) is a green very near the line E ; and (68) is a blue, about one- 

 third of the distance from F to G." 



A specimen observation is given : — 



" Oct. 18, J. 18-5(24) + 27(44) + 37(68) = W . 



This equation means that on the 18th of October the observer J (myself), 

 made an observation in which the breadth of the slit X was 18-5, as measured 

 by the wedge, while its centre was at the division (24) of the scale ; that the 

 breadths of Y and Z were 27 and 37, and their positions (44) and (68) ; and 

 that the illumination produced by these slits was exactly equal, in my estima- 

 tion as an observer, to the constant white W. 



" The position of the slit X was then shifted from (24) to (28), and when 

 the proper adjustments were made, I found a second colour-equation of this 

 form — 



Oct. 18, J. 16(28) + 21(44) + 37(68) = W . 



Subtracting one equation from the other, and remembering that the figures 

 in brackets are merely symbols of position, not of magnitude, we find 



16(28) = 18-5(24) + 6(44), 



showing that (28) can be made up of (24) and (44), in the proportion of 18-5 

 to 6. 



" In this way, by combining each colour with two standard colours, we may 

 produce a white equal to the constant white. The red and yellow colours from 

 (20) to (32) must be combined with green and blue, the greens from (36) to 

 (52) with red and blue, and the blue from (56) to (80) with red and green." 



The values employed in the present paper are those of Maxwell's second 

 observer K (whose vision in the region of the line F was more normal than his 

 own)*, and are given in his table No. VI. For our purpose they require some 

 extension, especially at the violet end. Thus the equivalents of (16), (84), (88), 

 (92), (96), (100), are obtained by a graphical extrapolation from the curves given 

 by Maxwell. The adjoining table is deduced from his with some reduction, in 

 order to exhibit the value, in terms of the three standards, of the illumination 

 due to the unit width of slit in each case. It will be seen that the extrapola- 

 tion at the upper end of the spectrum is necessary in order to make up 

 anything like the full total of (68). 



* It is understood that K represents Mrs Maxwell. In these matters a woman's observations are 

 generally to he preferred to a man's, as less liable to irregularities of the kind described in Nature, Nov. 

 17, 1881. 



