The phenomena are looked upon as due
to mechanical deformations or to movements governed by certain forces.
The electromagnetic theory leads, as we have seen, to the employment
of other images. M.H. Poincare, and, after him, Helmholtz, have both
proposed electromagnetic theories of dispersion. On examining things
closely, it will be found that there are not, in truth, in the two
ways of regarding the problem, two equivalent translations of exterior
reality. The electrical theory gives us to understand, much better
than the mechanical one, that _in vacuo_ the dispersion ought to be
strictly null, and this absence of dispersion appears to be confirmed
with extraordinary precision by astronomical observations. Thus the
observation, often repeated, and at different times of year, proves
that in the case of the star Algol, the light of which takes at least
four years to reach us, no sensible difference in coloration
accompanies the changes in brilliancy.
Sec. 2. THE THEORY OF LORENTZ
Purely mechanical considerations have therefore failed to give an
entirely satisfactory interpretation of the phenomena in which even
the simplest relations between matter and the ether appear.
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