ver, supervened, and the instrument
was not in his hands until October 16, 1868. On the 20th he picked up
the vivid rays, of which the presence and (approximately) the positions
had in the interim become known. But there is little doubt that, even
without that previous knowledge, they would have been found; and that
the eclipse of August 18 only accelerated a discovery already assured.

Sir William Huggins, meanwhile, had been tending towards the same goal
during two and a half years in his observatory at Tulse Hill. The
principle of the spectroscopic visibility of prominence-lines at the
edge of an uneclipsed sun was quite explicitly stated by him in
February, 1868,[517] and he devised various apparatus for bringing them
into actual view; but not until he knew where to look did he succeed in
seeing them.

Astronomers, thus liberated, by the acquisition of power to survey them
at any time, from the necessity of studying prominences during eclipses,
were able to concentrate the whole of their attention on the corona. The
first thing to be done was to ascertain the character of its spectrum.
This was seen in 1868 only as a faintly continuous one; for Rayet, who
seems to have perceived its distinctive bright line far above the
summits of the flames, connected it, nevertheless, with those objects.
On the other hand, Lieutenant Campbell ascertained on the same occasion
the polarisation of the coronal light in planes passing through the
sun's centre,[518] thereby showing that light to be, in whole or in
part, reflected sunshine. But if reflected sunshine, it was objected,
the chief at least of the dark Fraunhofer lines should be visible in it,
as they are visible in moonbeams, sky illumination, and all other
sun-derived light. The objection was well founded, but was prematurely
urged, as we shall see.

On the 7th of August, 1869, a track of total eclipse crossed the
continent of North America diagonally, entering at Behring's Straits,
and issuing on the coast of North Carolina. It was beset with observers;
but the most effective work was done in Iowa. At Des Moines, Professor
Harkness of the Naval Observatory, Washington, obtained from the corona
an "absolutely continuous spectrum," slightly less bright than that of
the full moon, but traversed by a single green ray.[519] The same green
ray was seen at Burlington and its position measured by Professor Young
of Dartmouth College.[520] It appeared to coincide with that of a d