300 HERSCHEL. 
tail was not flat as it appeared to be; it had the form of 
a conoid, with its sides of a certain thickness. The 
visual lines which traversed those sides almost tangenti- 
ally, evidently met much more matter than the visual 
lines passing across. This maximum of matter could not 
fail of being represented by a maximum of light. 
The luminous semi-ring floated; it appeared one day 
to be suspended in the diaphanous atmosphere by which 
the head of the comet was surrounded, at a distance of 
518,000 kilometres (822,000 English miles) from the 
nucleus. 
This distance was not constant. The matter of the 
semi-annular envelop 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 the tail seemed to undergo 
rapid, frequent, and considerable variations of length. 
Herschel discerned symptoms of a movement of rotation 
both in the comet and in its tail. This rotatory motion 
carried unequal shreds from the centre towards the bor- 
der, and reciprocally. On looking from time to time at 
the same region of the tail, at the border, for example, 
sensible changes of length must have been perceptible, 
which however had no reality in them. Herschel 
thought, as I have already said, that the beautiful comet 
of 1811, and that -of 1807, were self-luminous. The 
second comet of 1811 appeared to him to shine only by 
borrowed light. It must be acknowledged that these 
conjectures did not rest on any thing demonstrative. 
In attentively comparing the comet of 1807 with the 
