ts that I have examined, fourteen had no
solid body visible at their centre; the other two exhibited a central
light, very ill defined, that might be termed a nucleus, but a light
that certainly could not deserve the name of a disk."

The beautiful comet of 1811 became the object of that celebrated
astronomer's conscientious labour. Large telescopes showed him, in the
midst of the gazeous head, a rather reddish body of planetary
appearance, which bore strong magnifying powers, and showed no sign of
phase. 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 does not appear a demonstrative
argument.

The light of the head had a blueish-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 in 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 there 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 seemed dark, very rare,
and very diaphanous.

The luminous semi-ring always presented similar appearances in all the
positions of the comet; it was not then possible to attribute to it
really the annular form, the shape of Saturn's ring, for example.
Herschel sought whether a spherical demi-envelop of luminous matter, and
yet diaphanous, would not lead to a natural explanation of the
phenomenon. In this hypothesis, the visual rays, which on the 6th of
October, 1811, made a section of the envelop, or bore almost
tangentially, traversed a thickness of matter of about 399,000
kilometres, (248,000 English miles,) whilst the visual rays near the
head of the comet did not meet above 80,000 kilometres (50,000 miles) of
it. As the brightness must be proportional to the quantity of matter
traversed, there could not fail to be an appearance around the comet, of
a semi-ring five times more luminous than the central regions. This
semi-ring, then, was an effect of projection, and it has revealed a
circumstance to us truly remarkable in the physical constitution of
comets.

The two lumin