corresponding to the difference between the light of the nucleus
intercepted by the photosphere, and the light which issues from the
latter.

The man to whom we owe this noble generalisation is Kirchhoff,
Professor of Natural Philosophy in the University of Heidelberg;
[Footnote: Now Professor in the University of Berlin.] but, like
every other great discovery, it is compounded of various elements. Mr.
Talbot observed the bright lines in the spectra of coloured flames.
Sixteen years ago Dr. Miller gave drawings and descriptions of the
spectra of various coloured flames. Wheatstone, with his accustomed
ingenuity, analysed the light of the electric spark, and showed that
the metals between which the spark passed determined the bright bands
in the spectrum of the spark. Masson published a prize essay on these
bands; Van der Willigen, and more recently Plucker, have given us
beautiful drawings of the spectra, obtained from the discharge of
Ruhmkorff's coil. But none of these distinguished men betrayed the
least knowledge of the connection between the bright bands of the
metals and the dark lines of the solar spectrum. The man who came
nearest to the philosophy of the subject was Angstrom. In a paper
translated from Poggendorff's 'Annalen' by myself, and published in
the 'Philosophical Magazine' for 1855, he indicates that the rays
which a body absorbs are precisely those which it can emit when
rendered luminous. In another place, he speaks of one of his spectra
giving the general impression of a reversal of the solar spectrum.
Foucault, Stokes, and Thomson, have all been very close to the
discovery; and, for my own part, the examination of the radiation and
absorption of heat by gases and vapours, some of the results of which
I placed before you at the commencement of this discourse, would have
led me in 1859 to the law on which all Kirchhoff's speculations are
founded, had not an accident withdrawn me from the investigation. But
Kirchhoff's claims are unaffected by these circumstances. True, much
that I have referred to formed the necessary basis of his discovery;
so did the laws of Kepler furnish to Newton the basis of the theory of
gravitation. But what Kirchhoff has done carries us far beyond all
that had before been accomplished. He has introduced the order of law
amid a vast assemblage of empirical observations, and has ennobled our
previous knowledge by showing its relationship to some of the most
sublime