blished truths of science. The broad result, that the dark lines in
the spectrum of the sun afford an index to its chemical composition no
less reliable than any of the tests used in the laboratory, was equally
captivating to the imagination of the vulgar, and authentic in the
judgment of the learned; and, like all genuine advances in the knowledge
of Nature, it stimulated curiosity far more than it gratified it. Now
the history of how discoveries were missed is often quite as instructive
as the history of how they were made; it may then be worth while to
expend a few words on the thoughts and trials by which, in the present
case, the actual event was heralded.

Three times it seemed on the verge of being anticipated. The experiment,
which in Kirchhoff's hands proved decisive, of passing sunlight through
glowing vapours and examining the superposed spectra, was performed by
Professor W. A. Miller of King's College in 1845.[395] Nay, more, it was
performed with express reference to the question, then already (as has
been noted) in debate, of the possible production of Fraunhofer's lines
by absorption in a solar atmosphere. Yet it led to nothing.

Again, at Paris in 1849, with a view to testing the asserted coincidence
between the solar D-line and the bright yellow beam in the spectrum of
the electric arc (really due to the unsuspected presence of sodium),
Leon Foucault threw a ray of sunshine across the arc and observed its
spectrum.[396] He was surprised to see that the D-line was rendered more
intensely dark by the combination of lights. To assure himself still
further, he substituted a reflected image of one of the white-hot
carbon-points for the sunbeam, with an identical result. _The same ray
was missing._ It needed but another step to have generalised this
result, and thus laid hold of a natural truth of the highest importance;
but that step was not taken. Foucault, keen and brilliant though he was,
rested satisfied with the information that the _voltaic arc_ had the
power of stopping the kind of light emitted by it; he asked no further
question, and was consequently the bearer of no further intelligence on
the subject.

The truth conveyed by this remarkable experiment was, however, divined
by one eminent man. Professor Stokes of Cambridge stated to Sir William
Thomson (now Lord Kelvin), shortly after it had been made, his
conviction that an absorbing atmosphere of sodium surrounded the sun.
And so forcibly was h