the greatest
utility in astronomy of position, and the value of which enters into
electromagnetic theory, has to-day assumed, with the new ideas on the
constitution of matter, a still more considerable importance. I refer
to the speed of light, which appears to us, as we shall see further
on, the maximum value of speed which can be given to a material body.

After the historical experiments of Fizeau and Foucault, taken up
afresh, as we know, partly by Cornu, and partly by Michelson and
Newcomb, it remained still possible to increase the precision of the
measurements. Professor Michelson has undertaken some new researches
by a method which is a combination of the principle of the toothed
wheel of Fizeau with the revolving mirror of Foucault. The toothed
wheel is here replaced, however, by a grating, in which the lines and
the spaces between them take the place of the teeth and the gaps, the
reflected light only being returned when it strikes on the space
between two lines. The illustrious American physicist estimates that
he can thus evaluate to nearly five kilometres the path traversed by
light in one second. This approximation corresponds to a relative
value of a few hundred-thousandths, and it far exceeds those hitherto
attained by the best experimenters. When all the experiments are
completed, they will perhaps solve certain questions still in
suspense; for instance, the question whether the speed of propagation
depends on intensity. If this turns out to be the case, we should be
brought to the important conclusion that the amplitude of the
oscillations, which is certainly very small in relation to the already
tiny wave-lengths, cannot be considered as unimportant in regard to
these lengths. Such would seem to have been the result of the curious
experiments of M. Muller and of M. Ebert, but these results have been
recently disputed by M. Doubt.

In the case of sound vibrations, on the other hand, it should be noted
that experiment, consistently with the theory, proves that the speed
increases with the amplitude, or, if you will, with the intensity. M.
Violle has published an important series of experiments on the speed
of propagation of very condensed waves, on the deformations of these
waves, and on the relations of the speed and the pressure, which
verify in a remarkable manner the results foreshadowed by the already
old calculations of Riemann, repeated later by Hugoniot. If, on the
contrary, the amplitude is s