163 
the comet were made from the 4th of Oc- 
tober to the 19th. In all which time the 
comet uniformly preserved the appear- 
‘ance of a planetary disk fully enlightened 
by the sun: if was every where equally 
bright, round, and well defined on its bor- 
ders. Now as that part of the disk which 
was then visible to us could not possibly 
have a full illumination from the sun, f 
have calculated the phases of the comet 
for the 4th and for the 19th: the result of 
which is, that on the 4th the illumination 
was 119° 45' 9”, and that on the 19th it 
had gradually increased to 124° 22’ 40”. 
Both phases appear to me sutiiciently de-- 
falcated, to prove that the comet did not 
shine by light reflected from the sun only ; 
for, had this beer the case, the deficiency, 
} think, would have been perceived, not- 
withstanding the smallness of the object. 
‘Those who are acquainted with my expe- 
riments on small silver globules will easi- 
ly admit, that the same telescope which 
could show the spherical form of balls, 
that subtended only a few tenths of a se- 
cond in diameter, would surely not have 
represented a cometary disk as circular, 
#f ithad been as deficient as are the figures 
which give the calculated appearances. 
If these remarks are wel] founded we, 
are authorised to conclude, that the bedy 
of the comet on its surface is self-lumi- 
nous, from whatever cause this quality 
may be derived. The vivacity of the light 
of the comet also had a much greater re- 
semblance to the radiance of the stars, 
than to the mild reflection of the sun’s 
beams from the moon, which is an additi- 
* onal support to our former inference. 
The changes in the brightness of the 
small stars, when they are successively im- 
merged in the tail or coma of the comet, 
or clear from them, prove evidently, that 
they are sufficiently dense to obstract the 
free passage of star-light. Indeed if the 
tail or coma were composed of particles 
that reflect the light of the sun, to make 
them visible we ought rather to expect 
that the number of solid reflectiug parti~ 
cles, required for this purpose, would en- 
tirely prevent our seeing any stars through 
them. But the brightness of the head, 
* coma, and tail alone, will sufficiently ac 
count for the observed changes, if we ad- 
mit that they shine not by reflection, but 
_by their own radiance ; for a faint object 
projected on a bright ground, or seen 
through it, will certamly appear some- 
what fainter, although its rays should meet 
with no obstruction in coming to the eye. 
Now, as in this case we are sure of the 
bright interposition of the parts of the co« 
Proceedings of Learwted Societies. 
{March 1, 
met, but have no knowledge of fleating 
particles, we ought certainly not to as- 
cribe an effect to a hypothetical cause, 
when the existence of one, quite sufficient 
to explain the phenomena, is evident. ~ 
If we admit that the observed full illus 
mination of the disk of the comet cannot 
be accounted for from reflection, we may 
draw the same conclusion, with respect to 
the brightness of the head, coma, and tail,. 
from the following consideration. The 
observation of the 2d of February men- 
tions, that not only the head and coma 
were still very bright, but that also the 
faint remains of the tail were visible; but 
the distance of the comet from the Earth, 
atthe time of observation, was nearly 
240 millions of miles*, which proves, [ 
think, that no light reflected from floating 
particles could possibly have reached the 
eye, without supposing the number, ex- 
tent, and density of these particles far 
greater than what can be admitted. 
My last observation of the comet, on 
the 2ist of February, gives additional sup- _ 
port to what has been said; fur at the 
time of this observation the comet was- 
almost 2°9 times the mean distance of the 
sun from theearthf. It was also nearly 
2-7 from the sunf{. What chance then 
could rays going to the comet from the 
sun, at such a distance, have to be seen 
after reilectign, by an eye placed at more 
_than 275 millions of miles § frem the co- 
met? And yet the“instant the -comeét 
made its appearance in the telescope, it 
struck the eye as @ yery couspicuous ob- 
ject. 
The immense tails also of some comets 
that have been observed, and even that 
of the present one, the tail of which, on 
the 18th of October, was expanded over 
a space of more than nine millions of 
mailes |], may be accounted for more satis- 
factorily, by admitting them to consist of 
radiant matter, such as, for instance, the 
aurora borealis, taan when we unnécessa- 
rily ascxibe their light to a reflection of 
the sun’s tilumination thrown upon va- 
pours supposed te arise from the body of 
the comet. ; é A 
By the gradual increase of the distance 
of our comet, we have seen, that it assu- 
med the resemblance of a nebula; and it 
is certain, that had I met with it in one 
“s esasuuien... ee 
* 239894939. 
-t The sun’s mean distance being 1, that 
of the comet was 2°89797. ; 
T The comet’s distance from the sun was 
2°683196. 
§ 275077889. / 
lj 9160542. comes 
ef 
