d by "Balmer's Law," supplies a sure test for
discriminating, among newly discovered lines, those that appertain from
those that are unrelated to it. Deslandres' five additional
prominence-rays, for instance, were at once seen to make part of the
series, because conforming to its law;[1420] while a group of six dusky
bands, photographed by Sir William and Lady Huggins, April 4,
1890,[1421] near the extreme upper end of the spectrum of Sirius, were
pronounced without hesitation, for the opposite reason, to have nothing
to do with hydrogen. Their true affinities are still a matter for
inquiry.

As regards the hydrogen spectrum, however, the stars had further
information in reserve. Until recently, it was supposed to consist of a
single harmonic series, although, by analogy, three should co-exist. In
1896, accordingly, a second, bound to the first by unmistakable
numerical relationships, was recognised by Professor Pickering in
spectrographs of the 2.5 magnitude star Zeta Puppis,[1422] and the
identification was shortly afterwards extended to prominent Wolf-Rayet
emission lines. The discovery was capped by Dr. Rydberg's indication of
the Wolf-Rayet blue band at Lambda 4,688 as the fundamental member
of the third, and principal, hydrogen series.[1423] None of the
"Pickering lines" (as they may be called to distinguish them from the
"Huggins series") can be induced to glimmer in vacuum-tubes. They seem
to characterise bodies in a primitive state,[1424] and are in many cases
associated with absorption rays of oxygen, the identification of which
by Mr. McClean in 1897[1425] was fully confirmed by Sir David
Gill.[1426] The typical "oxygen star" is Beta Crucis, one of the
brilliants of the Southern Cross; but the distinctive notes of its
spectrum occur in not a few specimens of the helium class. Thus, Sir
William and Lady Huggins photographed several ultra-violet oxygen lines
in Beta Lyrae,[1427] and found in Rigel signs of the presence of
nitrogen,[1428] which, as well as silicium, proves to be a tolerably
frequent constituent of such orbs.[1429] For some unknown reason,
metalloids tend to become effaced, as metals, in the normal course of
stellar development, exert a more and more conspicuous action.

Dr. Scheiner's spectrographic researches at Potsdam in 1890 and
subsequently, exemplify the immense advantages of self-registration. In
a restricted section of the spectrum of Capella, he was enabled to
determine nearly three h