The length of wave corresponding to the first spark-gap of Hertz was about 6 metres, and the longest waves perceptible by the retina are 7/10 of a micron.[24] [Footnote 24: See footnote 3.] These radiations are so far apart that it is not astonishing that their properties have not a perfect similitude.

Thus phenomena like those of diffraction, which are negligible in the ordinary conditions under which light is observed, may here assume a preponderating importance.

To play the part, for example, with the Hertzian waves, which a mirror 1 millimetre square plays with regard to light, would require a colossal mirror which would attain the size of a myriametre[25] square.
[Footnote 25: I.e., 10,000 metres .-- ED.] The efforts of physicists have to-day, however, filled up, in great part, this interval, and from both banks at once they have laboured to build a bridge between the two domains.

We have seen how Rubens showed us calorific rays 60 metres long; on the other hand, MM.

Lecher, Bose, and Lampa have succeeded, one after the other, in gradually obtaining oscillations with shorter and shorter periods.