OF SOME OF THE STARS AND NEBTTLiE. 
563 
“The usual order of the phenomena which attend the formation of a tail appears to 
be that, as the comet approaches the sun, material is thrown off, at intervals, from the 
nucleus in the direction towards the sun. This material is not at once driven into the 
tail, but usually forms in front of the nucleus a dense luminous cloud, into which for a 
time the bright matter of the nucleus continues to stream. In this way a succession of 
envelopes may be formed, the material of which afterwards is dissipated in a direction 
opposite to the sun, and forms the tail. Between these envelopes dark spaces are 
usually seen. 
“ If the matter of the nucleus is capable of forming by condensation a cloud-like mass, 
there must be an intermediate state in which the matter ceases to be self-luminous, but 
yet retains its gaseous state, and reflects but little light. Such a non-luminous and 
transparent condition of the cometary matter may possibly be represented by some at 
least of the dark spaces which, in some comets, separate the cloud-like envelopes from 
the nucleus and from each other.” 
Now considerable differences of colour have been remarked in the different parts of 
some comets. The spectrum of this comet would show that its colour was bluish green. 
Sir W. Herschel described the head of the Comet of 1811 to be of a greenish or bluish- 
green colour, while the central point appeared to be of a pale ruddy tint. The repre- 
sentations of Halley’s comet at its appearance in 1835, by the elder Struve, are coloured 
bluish green, and the nucleus on October 9 is coloured reddish yellow. He describes 
the nucleus on that day, thus : — “ Her Kern zeigte sich wie eine kleine, etwas ins gelb- 
liche spielende, gluhende Kohle von langlicher Form”* *. Dr. Winnecke describes 
similar colours in the bright comet of 1862. “Die Farbe des Strahls erscheint mir 
gelbrothlich ; die des umgebenden Nebels (vielleicht aus Contrast) mattblaulich.” “ Die 
Farbe der Ausstromung erscheint mir gelblich ; die Coma hat blauliches Lichff’f . 
Now carbon, if incandescent in the solid state, or reflecting, when in a condition of 
minute division the light of the sun, would afford a light which, in comparison with that 
emitted by the luminous vapour of carbon, would appear as yellowish or approaching 
to red. 
The views of comets presented in this paper do not, however, afford any clue to the 
great mystery Which surrounds the enormous rapidity with which the tail is often pro- 
jected to immense distances. There are not any known properties peculiar to carbon, 
even when in a condition of extremely minute division, which would help to a solution 
of the enigma of the violent repulsive power from the sun which appears to be exerted 
upon cometary matter shortly after its expulsion from the nucleus, and upon matter 
tail is neither more or less than the accumulation of this sort of luminous vapour darted off, in th e first instance 
towards the sun, as it were something raised up, and, as it were, exploded by the sun’s heat out of the kernel, 
and then immediately and forcibly turned back and repelled from the sun.” — Sir John Herschel. Familiar 
Lectures on Scientific Subjects, p. 115. 
* Beobachtungen des Halleyschen Cometen, s. 41. 
f Memoires de l’Academie Imperiale des Sciences de St. Petersbourg, tome vii. No. 7. 
