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.