sally adopted. His new
table of standard solar lines was published in 1893.[668] Through his
work, indeed, knowledge of the solar spectrum so far outstripped
knowledge of terrestrial spectra, that the recognition of their common
constituents was hampered by intolerable uncertainties. Thousands of the
solar lines charted with minute precision remained unidentified for want
of a corresponding precision in the registration of metallic lines.
Rowland himself, however, undertook to provide a remedy. Aided by Lewis
E. Jewell, he redetermined, at the Johns Hopkins University, the
wave-lengths of about 16,000 solar lines,[669] photographing for
comparison with them the spectra of all the known chemical elements
except gallium, of which he could procure no specimen. The labour of
collation was well advanced when he died at the age of fifty-two, April
16, 1901. Investigations of metallic arc-spectra have also been carried
out with signal success by Hasselberg,[670] Kayser and Runge, O.
Lohse,[671] and others.

Another condition _sine qua non_ of progress in this department is the
separation of true solar lines from those produced by absorption in our
own atmosphere. And here little remains to be done. Thollon's great
Atlas[672] was designed for this purpose of discrimination. Each of its
thirty-three maps exhibits in quadruplicate a subdivision of the solar
spectrum under varied conditions of weather and zenith-distance.
Telluric effects are thus made easily legible, and they account wholly
for 866, partly for 246, out of a total of 3,200 lines. But the death of
the artist, April 8, 1887, unfortunately interrupted the half-finished
task of the last seven years of his life. A most satisfactory record,
meanwhile, of selective atmospheric action has been supplied by the
experiments and determinations of Janssen, Cornu and Egoroff, by Dr.
Becker's drawings,[673] and Mr. McClean's photographs of the analysed
light of the sun at high, low, and medium altitudes; and the autographic
pictures obtained by Mr. George Higgs, of Liverpool, of certain
rhythmical groups in the red, emerging with surprising strength near
sunset, excite general and well-deserved admiration.[674] The main
interest, however, of all these documents resides in the information
afforded by them regarding the chemistry of the sun.

The discovery that hydrogen exists in the atmosphere of the sun was made
by Angstrom in 1862. His list of solar elements published in that