3*8 



NATURE 



[Fed. 5, 1880 



Thomson in 1 S43 between the steady flow of heat and 

 the phenomena of statical electricity. 



Other five papers on the same subject were communi- 

 cated by him to the Philosophical Magazine in 1 86 1-2, 

 under the title Physical Lines of Force. Then in 1864 

 appeared his great paper " On a Dynamical Theory of 

 the Electromagnetic Field." This was inserted in the 

 Philosophical Transactions, and may be looked upon as 

 the first complete statement of the theory developed in 

 the treatise on Electricity and Magnetism. 



In recent years he came to the conclusion that such 

 analogies as the conduction of heat, or the motion of the 

 mass-less but incompressible fluid, depending as they do 

 on Laplace's equation, were best symbolised by the 

 quaternion notation with Hamilton's V operator ; and in 

 consequence, in his work on electricity, he gives the ex- 

 pressions for all the more important physical quantities 

 in their quaternion form, though without employing the 

 calculus itself in their establishment. I have discussed 

 in another place (Nature, vol. vii. p. 478) the various 

 important discoveries in this remarkable work, which of 

 itself is sufficient to secure for its author a foremost place 

 among natural philosophers. I may here state that the 

 main object of the work is to do away with " action at a 

 distance," so far at least as electrical and magnetic forces 

 are concerned, and to explain these by means of stresses 

 and motions of the medium which is required to account 

 for the phenomena of light. Maxwell has shown that, on 

 this hypothesis, the velocity of light is the ratio of the 

 electro-magnetic and electro-static units. Since this 

 ratio, and the actual velocity of light, can be determined 

 by absolutely independent experiments, the theory can 

 be put at once to an exceedingly severe preliminary test. 

 Neither quantity is yet fairly known within about 2 or 3 

 per cent., and the most probable values of each certainly 

 agree more closely than do the separate determinations 

 of either. There can now be little doubt that Maxwell's 

 theory of electrical phenomena rests upon foundations as 

 secure as those of the undulatory theory of light. But 

 the life-long work of its creator has left it still in its 

 infancy, and it will probably require for its proper 

 development the services of whole generations of mathe- 

 maticians. 



This was not the only work of importance to which he 

 devoted the greater part of his time while an undergra- 

 duate at Cambridge. For he had barely obtained his 

 degree before he read to the Cambridge Philosophical 

 Society a remarkable paper On the Transformation of 

 Surfaces by Bending, which appears in their Transactions 

 with the date March 1854. The subject is one which 

 had been elaborately treated by Gauss and other great 

 mathematicians, but their methods left much to be de. 

 sired from the point of view of simplicity. This Clerk- 

 Maxwell certainly supplied ; and to such an extent that 

 it is difficult to conceive that any subsequent investigator 

 will be able to simplify the new mode of presentation as 

 much as Maxwell simplified the old one. Many of his 

 results, also, were real additions to the theory ; especially 

 his treatment of the Lines of Bending. But the whole 

 matter is one which, except in its almost obvious elements, 

 it is vain to attempt to popularise. 



The next in point of date of Maxwell's greatest 

 works is his " Essay on the Stability of the Motion of 



Saturn's Rings," which obtained the Adams' Prize in the 

 University of Cambridge in 1857. This admirable 

 investigation was published as a pamphlet in 1859. 

 Laplace had shown in the Micanique Celeste that a 

 uniform solid ring cannot revolve permanently about a 

 planet ; for, even if its density were so adjusted as to 

 prevent its splitting, a slight disturbance would inevitably 

 cause it to fall in. Maxwell begins by finding what 

 amount of want of uniformity would make a solid ring 

 stable. He finds that this could be effected by a satellite 

 rigidly attached to the ring, and of about 4^ times its 

 mass : — but that such an arrangement, while not agreeing 

 with observation, would require extreme artificiality of 

 adjustment of a kind not elsewhere observed. Not only 

 so, but the materials, in order to prevent its behaving 

 almost like a liquid under the- great forces to which it is 

 exposed, must have an amount of rigidity far exceeding 

 that of any known substance. 



He therefore dismisses the hypothesis of solid rings, 

 and (commencing with that of a ring of equal and equi- 

 distant satellites) shows that a continuous liquid ring 

 cannot be stable, but may become so when broken up 

 into satellites. He traces in a masterly way the effects of 

 the free and forced waves which must traverse the ring, 

 under various assumptions as to its constitution ; and he 

 shows that the only system of rings which can dynamically 

 exist must be composed of a very great number of separate 

 masses, revolving round the planet with velocities depend- 

 ing on their distances from it. But even in this case the 

 system of Saturn cannot be permanent, because of the 

 mutual actions of the various rings. These mutual 

 actions must lead to the gradual spreading out of the 

 whole system, both inwards and outwards : — but if, as 

 is probable, the outer ring is much denser than the inner 

 ones, a very small increase of its external diameter would 

 balance a large change in the inner rings. This is con- 

 sistent with the progressive changes which have been 

 observed since the discovery of the rings. An ingenious 

 and simple mechanism is described, by which the motions 

 of a ring composed of equal satellites can be easily 

 demonstrated. 



Another subject which he treated with great success, 

 as well from the experimental as from the theoretical 

 point of view, was the Perception of Colour, the Primary 

 Colour Sensations, and the Nature of Colour Blindness. 

 His earliest paper on these subjects bears date 1855, and 

 the seventh has the date 1872. He received the Rumford 

 Medal from the Royal Society in i860, "For his Re- 

 searches 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 sub- 

 ject 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 + b\ + cZ. 

 Here a, b, c are numerical coefficients, which may be 



1 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 dciiiomtralcd. 



