is hearer impressed with the weight of the argument
based upon the absolute agreement of the D-line in the solar spectrum
with the yellow ray of burning sodium (then freshly certified by W. H.
Miller), combined with Foucault's "reversal" of that ray, that he
regularly inculcated, in his public lectures on natural philosophy at
Glasgow, five or six years before Kirchhoff's discovery, not only the
_fact_ of the presence of sodium in the solar neighbourhood, but also
the _principle_ of the study of solar and stellar chemistry in the
spectra of flames.[397] Yet it does not appear to have occurred to
either of these two distinguished professors--themselves among the
foremost of their time in the successful search for new truths--to
verify practically a sagacious conjecture in which was contained the
possibility of a scientific revolution. It is just to add, that
Kirchhoff was unacquainted, when he undertook his investigation, either
with the experiment of Foucault or the speculation of Stokes.

For C. J. Angstrom, on the other hand, perhaps somewhat too much has
been claimed in the way of anticipation. His _Optical Researches_
appeared at Upsala in 1853, and in their English garb two years
later.[398] They were undoubtedly pregnant with suggestion, yet made no
epoch in discovery. The old perplexities continued to prevail after, as
before their publication. To Angstrom, indeed, belongs the great merit
of having revived Euler's principle of the equivalence of emission and
absorption; but he revived it in its original crude form, and without
the qualifying proviso which alone gave it value as a clue to new
truths. According to his statement, a body absorbs all the series of
vibrations it is, under any circumstances, capable of emitting, as well
as those connected with them by simple harmonic relations. This is far
too wide. To render it either true or useful, it had to be reduced to
the cautious terms employed by Kirchhoff. Radiation strictly and
necessarily corresponds with absorption only _when the temperature is
the same_. In point of fact, Angstrom was still, in 1853, divided
between adsorption and interference as the mode of origin of the
Fraunhofer dark rays. Very important, however, was his demonstration of
the compound nature of the spark-spectrum, which he showed to be made up
of the spectrum of the metallic electrodes superposed upon that of the
gas or gases across which the discharge passed.

It may here be useful--