i ^6 Dr. Herschei^s Observations of a Comet , 
most accurately ascertained at the Royal Observatory at 
Greenwich. 
The different parts of a comet have been generally ex- 
pressed by terms that may be liable to misapprehension, such 
as the head, the tail, the coma, and the nucleus ; for in read- 
ing what some authors say of the head, when they speak of 
the size of the comet, it is evident that they take it for what 
is often called the nucleus. The truth is, that inferior teles- 
copes, which cannot show the real nucleus, will give a certain 
magnitude of the comet, which may be called its head ; it in- 
cludes all the very bright surrounding light ; nor is the name 
of the head badly applied, if we keep it to this meaning ; and 
since, with proper restriction, the terms which have been used 
may be retained, I shall give a short account of my observa- 
tions of the comet, as they relate to the above-mentioned par- 
ticulars, namely, the nucleus, the head, the coma, and the tail, 
without regarding the order of the time when they were 
made ; the date of each observation, however, will be added, 
that any person who may hereafter be in possession of more 
accurate elements of the comet's orbit, than those which I have 
at present, may repeat the calculations in order to obtain a 
more correct result. 
Of the Nucleus. 
From what has already been said, it will easily be under- 
stood that by the nucleus of the comet, I mean that part of 
the head which appears to be a condensed or solid body, and 
in which none of the very bright coma is included. It should 
be remarked, that from this definition it follows, that when the 
nucleus is very small, no telescope, but what has light and 
power in an eminent degree, will show it distinctly. 
