286 MR J. CLERK MAXWELL ON COLOUR 



and the addition of this sensation to any others cannot alter it in their estima- 

 tion. It is for them equivalent to black. 



Hence, if we draw lines through D in different directions, the colours belong- 

 ing to the line ought to differ only in intensity as seen by them, so that one of 

 them may be reduced to the other by the addition of black only. If we draw DW 

 and produce it, all colours on the u^per side of DW will be varieties of blue, and 

 those on the under side varieties of yellow, so that the line DW is a boundary 

 line between their two kinds of colour, blue and yellow being the names by which 

 they call them. • 



The accuracy of this theory will be evident from the comparison of the expe- 

 riments which I had an opportunity of making on Mr N. and Mr X. with each 

 other, and with measurements taken from the diagram No. 2, which was con- 

 structed from the observations of ordinary eyes only, the point D alone being 

 ascertained from a series of observations by Mr N. 



Taking the point 7, between R and B, it appears, by measurement of the lines 

 R 7 and B 7, that 7 corresponds to 



•07 B + -93R 



By measurement of W 7 and D 7, and correction by means of the coefficient of 

 W, and calling D black in the colour-blind language, 7 corresponds to 



•105 W + -895Bk 



Therefore, 

 By measurement, . . -93 R + 07 B = -105 W + -895 Bk \ 



By observation N.&X. together, -94 R+ -06 B = -10W+ -90 Bk I . . . . (5). 

 By X. alone, . . . -93R + -07B= 10 W+ -90 Bk j 



The agreement here is as near as can be expected. 



By a similar calculation with respect to the point 8, between B and G, 



By measurement, . -43 B + -57 G = -335 W + -665 Bk \ 



Observed by N. and X., -41B + -59 G= -34 W+ -66 Bk > . . . . (6). 



By X. alone, . . -42B + -58G= -32 W+ -68 Bk J 



We may also observe, that the line GD crosses RY. At the point of inter- 

 section we have — 



By calculation, . . -87 R + -13 Y = -34 G + -66 Bk ^ 



Observed by N. and X, -86 R + -14 Y = -40 G + -60 Bk ! _ (7) 

 X, -84R+-16 Y=-31G+-69Bk [ 

 X, -90R + -10 Y = -27 G+-73Bk J 



Here observations are at variance, owing to the decided colours produced af- 

 fecting the state of the retina, but the mean agrees well with calculation. 



Drawing the line BY, we find that it cuts lines through D drawn to every 



