^34? Herschei/s Observations of a second Comet , 
most likely to throw some light upon a subject which still re- 
mains involved in great obscurity. When the comet of which 
the observations have been given in this paper is compared 
with the preceding one, it will be found to be extremely dif- 
ferent. Its physical construction appears indeed to approach 
nearly to a planetary condition. In its magnitude it bears a 
considerable proportion to the size of the planets ; the dia- 
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; that 
is to say reflected from the sun. For were it of a phosphoric, 
self-luminous nature, we could hardly account for 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;* 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 
107 times ; 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 20th 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 atmo- 
sphere appears from its chevelure, which, though faint, was 
of considerable extent ; and the elasticity of this atmosphere 
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. 
» See Observations of the First Comet. 
