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Blue emissions of the Wolf-Rayet type then became occasionally prominent, in remarkable correspondence with the varying lustre of the star; finally, a band at Lambda 3969, found by Wright at Lick to characterise nebular spectra, assumed abnormal importance; and in July the nebular transformation might be said to be complete. Striking alterations of colour attended these spectral vicissitudes.
Rydberg's indication of the Wolf-Rayet blue band at Lambda 4,688 as the fundamental member of the third, and principal, hydrogen series. They seem to characterise bodies in a primitive state, and are in many cases associated with absorption rays of oxygen, the identification of which by Mr. McClean in 1897 was fully confirmed by Sir David Gill.
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, and the identification was shortly afterwards extended to prominent Wolf-Rayet emission lines. The discovery was capped by Dr.
Thirty-two Wolf-Rayet stars were investigated, spectroscopically and spectrographically, by Professor Campbell with the great Lick refractor in 1892-94; and several disclosed the singularity, already noticed by him in Gamma Argûs, of giving out mixed series, the members of which change from vivid to obscure with increase of refrangibility.
It is difficult to imagine by what chromospheric machinery this curious result can be produced. Alcyone in the Pleiades presents the same characteristic. Alone among the hydrogen lines, crimson C glows in its spectrum, while all the others are dark. Luminosity of the Wolf-Rayet kind is particularly constant, both in quantity and quality.
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