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.
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