of these vibrations is
exceedingly small, even in relation to the wave-length, small as these
last are. If, in fact, the amplitude of the vibrations acquired a
noticeable value in comparison with the wave-length, the speed of
propagation should increase with the amplitude. Yet, in spite of some
curious experiments which seem to establish that the speed of light
does alter a little with its intensity, we have reason to believe
that, as regards light, the amplitude of the oscillations in relation
to the wave-length is incomparably less than in the case of sound.

It has become the custom to characterise each vibration by the path
which the vibratory movement traverses during the space of a
vibration--by the length of wave, in a word--rather than by the
duration of the vibration itself. To measure wave-lengths, the methods
must be employed to which I have already alluded on the subject of
measurements of length. Professor Michelson, on the one hand, and MM.
Perot and Fabry, on the other, have devised exceedingly ingenious
processes, which have led to results of really unhoped-for precision.
The very exact knowledge also of the speed of the propagation of light
allows the duration of a vibration to be calculated when once the
wave-length is known. It is thus found that, in the case of visible
light, the number of the vibrations from the end of the violet to the
infra-red varies from four hundred to two hundred billions per second.
This gamut is not, however, the only one the ether can give. For a
long time we have known ultra-violet radiations still more rapid, and,
on the other hand, infra-red ones more slow, while in the last few
years the field of known radiations has been singularly extended in
both directions.

It is to M. Rubens and his fellow-workers that are due the most
brilliant conquests in the matter of great wave-lengths. He had
remarked that, in their study, the difficulty of research proceeds
from the fact that the extreme waves of the infra-red spectrum only
contain a small part of the total energy emitted by an incandescent
body; so that if, for the purpose of study, they are further dispersed
by a prism or a grating, the intensity at any one point becomes so
slight as to be no longer observable. His original idea was to obtain,
without prism or grating, a homogeneous pencil of great wave-length
sufficiently intense to be examined. For this purpose the radiant
source used was a strip of platinum covered w