144 PROFESSOR PIAZZI SMYTH ON COMETARY PHYSICS. 
great length and small breadth of the comet itself. The first day that I saw the 
comet, three days after perihelio, it was not visible; but, clearly seen on every sub- 
sequent day, it existed until the whole was lost in faintness. 
The observation of these bodies is not, however, as hinted above, in a suffi- 
ciently satisfactory state for rigidly testing any calculable theory. This depends not 
only on the rarity of the appearance of comets (a matter beyond our control) ; but 
also on the insufficient means with which, and the untoward circumstances under 
which they are generally observed. The telescopes usually turned upon them, 
have been of so small an optical power, that they would have been considered 
utterly incompetent for ascertaining the nature of nebulze high up in the sky ; how 
much more so, when employed on nebulous objects close to the horizon, as the 
comets usually are, flickering and faint in vapour and smoke, and almost over- 
powered by the strength of the twilight. 
But a sufficiently powerful telescope need not any longer be a difficulty, since 
the publication of the inventions of the Eart of Rosse, and Messrs LassEt and 
NaAsmytu ; and the effect of the vapour of the horizon, and the glare of twilight, 
might be successfully overcome, by establishing an observatory on high land 
within the tropics, where the geographical position renders the twilight short 
even to the plains; and the rarity of mountain air would still further reduce the 
reflective power of the atmosphere. Micrometrical measures, with such instru- 
ments, and under such circumstances, should be combined with photometrical deter- 
minations of the brightness of the various parts of the comet, and of the background 
of the sky. The former observations are easy and straight forward enough, but 
the latter are difficult and new; the zero must inevitably be taken from a stellar 
scale, but none such exists at present ; for the telescope measure has invariably 
failed whenever employed for the purpose, and the eye is still thought the best 
available mean. Hence, none but the brightest stars have had their magni- 
tudes determined, and that but coarsely, while the great question still remains 
in much the same state as that in which the application of the telescope to divided 
instruments was in, before men had learnt how to determine the error of colli- 
mation. They knew that there were vast powers of accuracy in the optic tubes, 
but were afraid of great and mysterious errors, which they neither exactly 
understood, nor saw how to correct. Similarly in photometry, a telescope of 
large aperture, is confessed to have a larger scale and range than the unassisted 
eye, but is suspected of misleading to a greater extent. 
This is hardly the place for entering into such an experimental branch of 
practical astronomy ; for pointing out what appears to be the error of the methods 
adopted by others; and for shewing the correctness and efficacy, as I believe, of 
another plan, which might be adapted to telescopes of any size. But there can 

