38 On the Tails of Comets. 
the proportion of the square of the diminution of distance, —~ 
again that the tail should have a cylindrical and hollow appear- 
ance, the rays of light being at least partially obstructed by the nu- 
cleus; moreover, that the tail should be curved, by the necessary 
affott of aberration. I apprehend it will be acknowledged that the 
weight of testimony is decidedly favorable to the fact that the nu- 
clei of comets, though they generally resemble planets in form and 
brilliancy, may not be solid or opaque, inasmuch as some are wn- 
t 
; 
questionably oa and the quantity of matter in all is ex- ~ 
ceedingly inconsiderable 
Professor Struve saw a ested of the eleventh magnitude through - ‘ag 
the Encke comet; Sir William Herschel noticed one of the 
sixth magnitude through the centre of the comet of 1795; and _ 
his illustrious son, in a memoir communicated to the Royal As- 
tronomical Society, mentions that he saw a cluster of stars of the 
sixteenth magnitude very near the centre of Biela’s comet. Not- 
withstanding this tenuity, an increased density may always be 
noticed toward the centre of the head, except ina few small 
‘comets unaccompanied with trains. ai ae 
Astronomers of all ages seem to have been inclined toa baie 
in an ethereal medium, and the present one has afforded a conclu a 
sive evidence of its existence, in its effect upon the duration of — 
the revolution of the Encke comet. Professor Encke in -a-dis- 4 
sertation on this subject, after giving the minutiz of his observa-_— 
tions, very modestly remarks—“ If I may be permitted to express _ 
my opinion on a subject which for twelve years has inceaeail 
occupied me, in treating which I have avoided no method, now : 
ever circuitous, no kind of verification, in order to reach the tr # 
so far as it lay in my power; Ican not consider it otherwise than 
completely established, that an extraordinary connection is neces- 
sary for Pon’s* comet, and equally certain that the principal part 
of it consists in an increase of the mean motion proportionate to 
the time.” Professor Airy, in an appendix to a translation © 
Encke’s memoir, adds—“I can not but express my belief that 
the principal point of the theory, namely, an effect exactly sim- 
ilar to that which a resisting medium would produce, is ean! 
established by the reasoning of Professor Encke.” Arago, 10 
“=. of the discrepancy between the result of calculation and © 
epee 
vate 
? 
ee * Called by others Encke’s comet. 
