pped 324, while a few of the most
conspicuous he set up (if we may be permitted the expression) as
landmarks, measuring their distances apart with a theodolite, and
affixing to them the letters of the alphabet, by which they are still
universally known. Nor did he stop here. The same system of examination
applied to the rest of the heavenly bodies showed the mild effulgence of
the moon and planets to be deficient in precisely the same rays as
sunlight; while in the stars it disclosed the differences in likeness
which are always an earnest of increased knowledge. The spectra of
Sirius and Castor, instead of being delicately ruled crosswise
throughout, like that of the sun, were seen to be interrupted by three
massive bars of darkness--two in the blue and one in the green;[380] the
light of Pollux, on the other hand, seemed precisely similar to sunlight
attenuated by distance or reflection, and that of Capella, Betelgeux,
and Procyon to share some of its peculiarities. One solar line
especially--that marked in his map with the letter D--proved common to
all the four last-mentioned stars; and it was remarkable that it exactly
coincided in position with the conspicuous yellow beam (afterwards, as
we have said, identified with the light of glowing sodium) which he had
already found to accompany most kinds of combustion. Moreover, both the
_dark_ solar and the _bright_ terrestrial "D lines" were displayed by
the refined Munich appliances as double.

In this striking correspondence, discovered by Fraunhofer in 1815, was
contained the very essence of solar chemistry; but its true significance
did not become apparent until long afterwards. Fraunhofer was by
profession, not a physicist, but a practical optician. Time pressed; he
could not and would not deviate from his appointed track; all that was
possible to him was to indicate the road to discovery, and exhort others
to follow it.[381]

Partially and inconclusively at first this was done. The "fixed lines"
(as they were called) of the solar spectrum took up the position of a
standing problem, to the solution of which no approach seemed possible.
Conjectures as to their origin were indeed rife. An explanation put
forward by Zantedeschi[382] and others, and dubiously favoured by Sir
David Brewster and Dr. J. H. Gladstone,[383] was that they resulted from
"interference"--that is, a destruction of the motion producing in our
eyes the sensation of light, by the superposition of tw