radiations of tungsten. There must be twelve more of these edges between mercury (N = 80) and uranium (
) and De Broglie has measured them up to thorium (
). They become, however, very difficult to locate in this
region of frequencies on account of their extreme closeness to the central image. But the
radiations, which are of seven times longer wave-length, may then be used, and [Fig. 23] of the plate opposite this page shows the