in other words, taking up a
perfectly definite position in the spectrum. His experiments were
repeated by Morgan,[369] Wollaston, and--with far superior precision and
diligence--by Fraunhofer.[370] The great Munich optician, whose work was
completely original, rediscovered Melvill's deep yellow ray and measured
its place in the colour-scale. It has since become well known as the
"sodium line," and has played a very important part in the history of
spectrum analysis. Nevertheless, its ubiquity and conspicuousness long
impeded progress. It was elicited by the combustion of a surprising
variety of substances--sulphur, alcohol, ivory, wood, paper; its
persistent visibility suggesting the accomplishment of some universal
process of nature rather than the presence of one individual kind of
matter. But if spectrum analysis were to exist as a science at all, it
could only be by attaining certainty as to the unvarying association of
one special substance with each special quality of light.

Thus perplexed, Fox Talbot[371] hesitated in 1826 to enounce this
fundamental principle. He was inclined to believe that the presence in
the spectrum of any individual ray told unerringly of the volatilisation
in the flame under scrutiny of some body as whose badge or distinctive
symbol that ray might be regarded; but the continual prominence of the
yellow beam staggered him. It appeared, indeed, without fail where
sodium _was_; but it also appeared where it might be thought only
reasonable to conclude that sodium _was not_. Nor was it until thirty
years later that William Swan,[372] by pointing out the extreme delicacy
of the spectral test, and the singularly wide dispersion of sodium, made
it appear probable (but even then only probable) that the questionable
yellow line was really due invariably to that substance. Common salt
(chloride of sodium) is, in fact, the most diffusive of solids. It
floats in the air; it flows with water; every grain of dust has its
attendant particle; its absolute exclusion approaches the impossible.
And withal, the light that it gives in burning is so intense and
concentrated, that if a single grain be divided into 180 million parts,
and one alone of such inconceivably minute fragments be present in a
source of light, the spectroscope will show unmistakably its
characteristic beam.

Amongst the pioneers of knowledge in this direction were Sir John
Herschel[373]--who, however, applied himself to the subject in t