, 1, if the Moseley law were exact we should obtain, by solving for the wave-length of the highest frequency line which can be emitted by the element whose nucleus contains but one single positive electron. The result of this substitution is
(millionths millimeters). Now the wave-length corresponding to the highest observed frequency in the ultra-violet series of hydrogen lines recently discovered by Lyman is
and there is every reason to believe from the form of this series that its convergence wave-length—this corresponds to the highest frequency of which the hydrogen atom is theoretically capable—is
. The agreement is only approximate, but it is as close as could be expected in view of the lack of exact equality in the Moseley steps. It is well-nigh certain, then, that this Lyman ultra-violet series of hydrogen lines is nothing but the
X-ray series of hydrogen. Similarly, it is equally certain that the
