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 ypper 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 y, between R and B, it appears, by measurement of the lines 
R yand BY, that Y corresponds to 
07 B+:93 R 
By measurement of Wy and D7, and correction by means of the coefficient of 
W, and calling D black in the colour-blind language, Y corresponds to 
105 W + °895 Bk 
Therefore, 
By measurement, 5 . ‘93 R+-07 B=:105 W +-895 Bk 
By observation N. & X. together, ‘94 R+-06 B= 10 W+ :90 Bk (5). 
By X.alone, . . . ‘93R+07 B= 10 W+ 90 Bk 
The agreement here is as near as can be expected. 
By a similar calculation with respect to the point 6, between B and G, 
By measurement, . ‘43 B+°57 G=:335 W +:665 Bk 
Observed by N. and X., -41 B+°59 G= :34 W+ -66 Bk (6). 
By X. alone, . - 42 B+°'58 G= -32 W+ ‘68 Bk 
We may also observe, that the line GD crosses RY. At the point of inter- 
section we have— 
By ecaleulation, . 87 R+:13 Y=-34 G+-66 Bk | 
Observed iy N. and x. 86 R+:14 Y=-40 G+'60 Bk ; Ae 
es Xs, 84 R+-16 Y=:31 G+-69 Bk | 
X., 90 R+-10 Y=-27 G+-73 Bk 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 
