336 Proceedings of the Royal Society 
“ For his Researches on the Composition of Colours, and other 
optical papers.” Though a triplicity about colour had long been 
known or suspected, which Young had (most probably correctly) 
attributed to the existence of three sensations, and Brewster had 
erroneously* supposed to be objective, Maxwell was the first to 
make colour-sensation the subject of actual measurement. He proved 
experimentally that any colour C (given in intensity of illumination 
as well as in character) may be expressed in terms of three arbitrarily 
chosen standard colours, X, Y, Z, by the formula 
C = aX + bY + cZ. 
Here a , b, c are numerical coefficients, which may be positive or 
negative ; the sign = means “ matches,” + means “ superposed,” 
and - directs the term to be taken to the other side of the 
equation. 
The last of his greatest investigations bore on the Kinetic 
Theory of Gases. Originating with D. Bernoulli, this theory was 
advanced by the successive labours of Herapath, Joule, and particu- 
larly of Clausius, to such an extent as to put its general accuracy 
beyond a doubt. But by far the greatest developments it has 
received are due to Maxwell, part of whose mathematical work has 
recently been still further extended in some directions by Boltzmann. 
In this field Maxwell appears as an experimenter (on the laws of 
gaseous friction) as well as a mathematician. His two latest 
papers deal with this branch of physics; one is an extension 
and simplification of some of Boltzmann’s chief results, the other 
treats of the kinetic theory as applied to the motion of the radio- 
meter. 
He has written an admirable text-book of the “ Theory of Heat,” 
which has already gone through several editions, and a very excel- 
lent elementary treatise on “ Matter and Motion.” (See, again, 
“Hature,” vol. xvi. p. 119.) Even this, like his other and larger 
works, is full of valuable matter, worthy of the most attentive 
perusal not of students alone but of the very foremost scientific 
men. 
* All we can positively say to be erroneous is some of the principal argu- 
ments by which Brewster’s view was maintained, for the subjective character 
of the triplicity has not been absolutely demonstrated. 
