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XX
LIGHT AND ELECTRICITY
could be other than elastic waves. Any attempt at a thorough
discussion of the one or the other of these assertions seemed
almost to be idle speculation. All the more must we admire
the happy genius of the man who could connect together these
apparently remote conjectures in such a way that they mutually
supported each other, and formed a theory of which every one
was at once bound to admit that it was at least plausible.
This was an Englishman-Maxwell.
You know the paper
which he published in 1865 upon the electromagnetic theory
of light. It is impossible to study this wonderful theory with-
out feeling as if the mathematical equations had an independent
life and an intelligence of their own, as if they were wiser than
ourselves, indeed wiser than their discoverer, as if they gave
forth more than he had put into them. And this is not alto-
gether impossible: it may happen when the equations prove
to be more correct than their discoverer could with certainty
have known. It is true that such comprehensive and accurate
equations only reveal themselves to those who with keen in-
sight pick out every indication of the truth which is faintly
visible in nature. The clue which Maxwell followed is well
known to the initiated. It had attracted the attention of
other investigators: it had suggested to Riemann and Lorenz
speculations of a similar nature, although not so fruitful in
results. Electricity in motion produces magnetic force, and
magnetism in motion produces electric force; but both of
these effects are only perceptible at high velocities. Thus
velocities appear in the mutual relations between electricity
and magnetism, and the constant which governs these relations
and continually recurs in them is itself a velocity of exceed-
ing magnitude. This constant was determined in various
ways, first by Kohlrausch and Weber, by purely electrical
experiments, and proved to be identical, allowing for the
experimental errors incident to such a difficult measurement,
with another important velocity-the velocity of light. This
might be an accident, but a pupil of Faraday's could scarcely
regard it as such. To him it appeared as an indication that
the same ether must be the medium for the transmission of
both electric force and light. The two velocities which were
found to be nearly equal must really be identical. But in
that case the most important optical constants must occur in