to the circumstance of the stars being projected on a luminous
background. Such was Herschel's explanation. A gaseous medium, capable
of absorbing sufficient solar radiance to efface the light of some
"lesser stars," appeared to him to possess in each stratum a sensible
quantity of matter. Hence it would cause a real diminution of the light
transmitted, though nothing would indicate the existence of such a
cause.[1]

[Footnote 1: This conclusion is disputed by many astronomers.]

Herschel examined the beautiful comet of 1811 with equal accuracy.
"Large telescopes showed him, in the midst of the gaseous head, a rather
reddish body of planetary appearance, which bore strong magnifying
powers, and showed no sign of _phase_ (that is, of change of aspect, as
in the case of the Moon). Hence Herschel concluded that it was
self-luminous. Yet, if we reflect that the planetary body under
consideration was not a second in diameter, the absence of a phase,"
says Arago, "does not appear a demonstrative argument."

The same writer adds:--

"The light of the head had a bluish-green tint." Was this a
real tint, or did the central reddish body, only through
contrast, make the surrounding vapour appear to be coloured?
Herschel did not examine the question from this point of view.

"The head of the comet appeared to be enveloped at a certain
distance, on the side towards the Sun, by a brilliant narrow
zone, embracing about a semicircle, and of a yellowish colour.
From the two extremities of the semicircle arose, towards the
region away from the Sun, two long luminous streaks which
limited the tail. Between the brilliant circular semi-ring and
the head, the cometary substance appeared to be dark, of great
rarity, and very diaphanous.

"The luminous self-ring floated: one day it seemed to be
suspended in the diaphanous atmosphere by which the head of the
comet was surrounded, at a distance of 322,000 English miles
from the nucleus.

"This distance was not constant. The matter of the semi-annular
envelope seemed even to be precipitated by slow degrees through
the diaphanous atmosphere; finally, it reached the nucleus; the
earlier appearances vanished; the comet was reduced to a
globular nebula.

"During its period of dissolution, the ring appeared sometimes
to have several branches.

"The luminous shreds of th