one who will
allow, on one hand, that the nucleus is a solid body, and on the other,
that it would have been possible to observe a phase of 8/10 on a disk
whose apparent total diameter did not exceed one or two seconds of a
degree.

Very small stars seemed to grow much paler when they were seen through
the coma or through the tail of the comet.

This faintness may have only been apparent, and might arise from the
circumstance of the stars being then projected on a luminous background.
Such is, indeed, the explanation adopted by Herschel. A gaseous medium,
capable of reflecting sufficient solar light to efface that of some
stars, would appear to him to possess in each stratum a sensible
quantity of matter, and to be, for that reason, a cause of real
diminution of the light transmitted, though nothing reveals the
existence of such a cause.

This argument, offered by Herschel in favour of the system which
transforms comets into self-luminous bodies, has not, as we may
perceive, much force. I might venture to say as much of many other
remarks by this great observer. He tells us that the comet was very
visible in the telescope on the 21st of February, 1808; now, on that
day, its distance from the sun amounted to 2.7 times the mean radius of
the terrestrial orbit; its distance from the observer was 2.9: "What
probability would there be that rays going to such distances, from the
sun to the comet, could, after their reflection, be seen by an eye
nearly three times more distant from the comet than from the sun?"

It is only numerical determinations that could give value to such an
argument. By satisfying himself with vague reasoning, Herschel did not
even perceive that he was committing a great mistake by making the
comet's distance from the observer appear to be an element of
visibility. If the comet be self-luminous, its intrinsic splendour (its
brightness for unity of surface) will remain constant at any distance,
as long as the subtended angle remains sensible. If the body shines by
borrowed light, its brightness will vary only according to its change of
distance from the sun; nor will the distance of the observer occasion
any change in the visibility; always, let it be understood, with the
restriction that the apparent diameter shall not be diminished below
certain limits.

Herschel finished his observations of a comet that was visible in
January, 1807, with the following remark:--

"Of the sixteen telescopic come