year,[675] the result of an investigation separate from, though
conducted on the same principle as Kirchhoff's, included the substance
which we now know to be predominant among them. Dr. Pluecker of Bonn had
identified in 1859 the Fraunhofer line F with the green ray of hydrogen,
but drew no inference from his observation. The agreement was verified
by Angstrom; two further coincidences were established; and in 1866 a
fourth hydrogen line in the extreme violet (named _h_) was detected in
the solar spectrum. With Thalen, he besides added manganese, titanium,
and cobalt to the constituents of the sun enumerated by Kirchhoff, and
raised the number of identical rays in the solar and terrestrial spectra
of iron to no less than 460.[676]

Thus, when Sir Norman Lockyer entered on that branch of inquiry in 1872,
fourteen substances were recognised as common to the earth and sun.
Early in 1878 he was able to increase the list provisionally to
thirty-three,[677] all except hydrogen metals. This rapid success was
due to his adoption of the test of _length_ in lieu of that of
_strength_ in the comparison of lines. He measured their relative
significance, in other words, rather by their persistence through a wide
range of temperature, than by their brilliancy at any one temperature.
The distinction was easily drawn. Photographs of the electric arc, in
which any given metal had been volatilised, showed some of the rays
emitted by it stretching across the axis of the light to a considerable
distance on either side, while many others clung more or less closely to
its central hottest core. The former "long lines," regarded as certainly
representative, were those primarily sought in the solar spectrum; while
the attendant "short lines," often, in point of fact, due to foreign
admixtures, were set aside as likely to be misleading.[678] The
criterion is a valuable one, and its employment has greatly helped to
quicken the progress of solar chemistry.

Carbon was the first non-metallic element discovered in the sun. Messrs.
Trowbridge and Hutchins of Harvard College concluded in 1887,[679] on
the ground of certain spectral coincidences, that this protean substance
is vaporised in the solar atmosphere at a temperature approximately that
of the voltaic arc. Partial evidence to the same effect had earlier been
alleged by Lockyer, as well as by Liveing and Dewar; and the case was
rendered tolerably complete by photographs taken by Kayser and Ru