CONSTRUCTION OF COMETS, &C, 
197 
meter of its nucleus being very nearly one-third of that of the 
earth. 
The light by which we see it is probably also planetary ; jt s light a p- 
that is to say, reflected from the sun. For were it of a phos- peared to be 
reflected or 
phoric, self-luminous nature, we could hardly account for pi anet;ar y. 
its little density : for instance, the very small body of the 
first comet, at the distance of 114 millions of miles from the 
earth, bore a magnifying power of 600 , and was even seen 
better with this than with a lower one 5 * whereas the second, 
notwithstanding its large size, and being only at the distance 
of 103 millions, had not light enough to bear conveniently to 
be magnified 10 / times j but if we admit this nucleus to be 
opaque, like the bodies of the planets, and of a nature not 
to reflect much light, then its distance from the sun, which the 
20 th of January was above 174 millions of miles, will explain 
the cause of its feeble illumination. 
That the nucleus of this comet was surrounded by an atmos- jt s atmos- 
phere appears from its chevelure, which, though faint, was pherewascon* 
of considerable extent : and the elasticity of this atmosphere S1 ‘ ’ 
may be inferred from the spherical figure of the chevelure, 
proved by its roundness and equal decrease of light at equal 
distances from the centre. 
The transparency of the atmosphere is partly ascertained and trans- 
from our seeing the nucleus through it, but may also be in- parent, 
ferred by analogy from an observation of the first comet. It 
will be remembered that an atmosphere of great transparency, 
which had been seen for a long time, was lost when the comet 
receded from the sun, by the subsidence of some nebulous 
matter not sufficiently rarified to enter the regions of the 
tail.f Now as the existence of this atmosphere, when it was 
no longer visible, might have been doubted, the luminous 
matter suspended in it, which had already 20 days obstructed 
our view of it, happened fortunately to be once more elevated 
the 9 th of December, and thereby enabled us, from its trans- 
parency and capacity of sustaining luminous vapours, to ascer- 
tain the continuance of its existence. By analogy, therefore, 
we may surmise that the faint chevelure of the second comet 
* See Observations of the First Comet, 
f See Observations of the First Comet. 
consists 
