Besides his enormous achievements in mathematics, Euler was deeply involved in many
areas of physics. His early work on music had a direct bearing on his study of sound,
which in due course contributed to his studies of the mechanics of continuous bodies, the
transmitters of sound vibrations. These important advances in continuum and fluid mechan-
ics also moved Euler to advocate a wave theory of light, as against Newton ’ s emission
(particle) theory. Throughout, Euler used the examples of sound and music as exemplars
for a new understanding of light and color.
In the century after his seminal work on optics, Newton ’ s theories remained the locus
of considerable controversy. On the Continent, his work found supporters as well as
notable critics. Leibniz, for one, was impressed but advocated careful repetition of New-
ton ’ s experiments. The prolific Christian Wolff became the chief popularizer of Newton ’ s
theory in German lands, but many scholars were more attracted to versions of Descartes ’ s
theory of a vibrating medium or ether that pervaded space.^1 Nor can such medium theories
be too sharply distinguished from Newton ’ s, who also argued for a “ subtle ether ” that
would fill space but not retard the heavenly bodies, though he considered light an emission
phenomenon rather than a state of the ether as such.
Others within the Cartesian tradition took the idea of a light-bearing medium in quite
different directions. For instance, in 1690 Christiaan Huygens considered light to be a
sequence of pulses traveling at a finite velocity within the medium. The word “ pulse ” here
should be distinguished from “ wave ” because Huygens dismissed the possibility that the
pulses follow each other at regular intervals, as would wave fronts. Yet some of his con-
cepts carried over to the later wave theory, most notably the formation of collective fronts
of pulses or waves through what now is called Huygens ’ s principle ( figure 10.1 ). Though
some accepted Huygens ’ s theory because of its account of the perplexing phenomenon of
double refraction — the passage of two different light rays at two different speeds through
a calcite crystal — others had trouble with his account of how simple rectilinear propagation
could be reconciled with ever-spreading circular pulse fronts. Most troubling, his theory
gave no account of colors; in the half century after 1700, it fell from sight in research
publications, though it was noted favorably in German textbooks.
barré
(Barré)
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