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of Edinburgh, Session 1879 - 80 . 
depending as they do on Laplace’s equation, were best symbolised 
by the quaternion notation with Hamilton’s v operator ; and in conse- 
quence, in his work on electricity, he gives the expressions 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 im- 
portant discoveries in this remarkable work, which of itself is suffi- 
cient to secure for its author a foremost place among natural philo- 
sophers. 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 mag- 
netic 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 both certainly agree niore closely than do the 
separate determinations of either. There can now be little doubt 
that Maxwell’s theory of electrical phenomena rests upon founda- 
tions 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 mathematicians. 
The next in point of date of Maxwell’s greatest works is his 
“Essay on the Stability of Saturn’s Rings,” which obtained the Adams’ 
Prize in 1859. In this admirable investigation he shows that it is 
dynamically impossible that these rings can be solid, and also that 
they cannot be continuous liquid masses ; the only other available 
hypothesis, viz., that they consist of multitudes of discrete parts, 
each a satellite, must therefore be the correct one. 
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 1860, 
