ous streaks that outlined the tail at its two limits, may
be explained in a similar manner; the 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
tangentially, 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 (322,000 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 border, 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 beautiful comet of
1811, relative to the changes of distance from the sun, and the
modifications resulting thence, Herschel put it beyond doubt that these
modifications have something individual in them, something relative to a
special state of the nebulous matter. On one celestial body the changes
of distance produce an enormous effect, on another the modifications are
insignificant.

OPTICAL LABOURS.

I shall say very little on the discoveries that Herschel made in
physics. In short, everybody knows them. They have been inserted into
special treatises, into elementary works, into verbal instruction; they
must be considered as the starti