o light-waves in
such a manner that the crests of one exactly fill up the hollows of the
other. This effect was supposed to be brought about by imperfections in
the optical apparatus employed.

A more plausible view was that the atmosphere of the earth was the agent
by which sunlight was deprived of its missing beams. For a few of them
this is actually the case. Brewster found in 1832 that certain dark
lines, which were invisible when the sun stood high in the heavens,
became increasingly conspicuous as he approached the horizon.[384] These
are the well-known "atmospheric lines;" but the immense majority of
their companions in the spectrum remain quite unaffected by the
thickness of the stratum of air traversed by the sunlight containing
them. They are then obviously due to another cause.

There remained the true interpretation--absorption in the _sun's_
atmosphere; and this, too, was extensively canvassed. But a remarkable
observation made by Professor Forbes of Edinburgh[385] on the occasion
of the annular eclipse of May 15, 1836, appeared to throw discredit upon
it. If the problematical dark lines were really occasioned by the
stoppage of certain rays through the action of a vaporous envelope
surrounding the sun, they ought, it seemed, to be strongest in light
proceeding from his edges, which, cutting that envelope obliquely,
passed through a much greater depth of it. But the circle of light left
by the interposing moon, and of course derived entirely from the rim of
the solar disc, yielded to Forbes's examination precisely the same
spectrum as light coming from its central parts. This circumstance
helped to baffle inquirers, already sufficiently perplexed. It still
remains an anomaly, of which no satisfactory explanation has been
offered.

Convincing evidence as to the true nature of the solar lines was however
at length, in the autumn of 1859, brought forward at Heidelburg.
Kirchhoff's _experimentum crucis_ in the matter was a very simple one.
He threw bright sunshine across a space occupied by vapour of sodium,
and perceived with astonishment that the dark Fraunhofer line D, instead
of being effaced by flame giving a luminous ray of the same
refrangibility, was deepened and thickened by the superposition.

He tried the same experiment, substituting for sunbeams light from a
Drummond lamp, and with similar result. A dark furrow, corresponding in
every respect to the solar D-line, was instantly seen to interrupt