28 6 
Dr . Brinkley on the 
said, that these uncertainties tend to correct each other, and 
that the uncertainty remaining, after taking a mean of re- 
sults from several stars, will be too small to be regarded. 
This indeed may be said as to the index error when applied 
to observations of the sun, moon, or planets ; but not, I think, 
when it is applied to investigations relative to the parallax 
of the fixed stars, annual variation of north polar distance, 
exact determination of the quantity of aberration and nutation, 
and these, it' will be allowed, are objects of great importance 
in the present improved state of astronomy. 
Indeed, with respect to the parallax of the fixed stars,' 
several of these objections may be obviated by a proper 
selection of the standard stars. Thus the uncertainties of 
refraction may be avoided by using only stars near the zenith. 
The objection in the 8th article may be partly obviated by 
using the same stars for ascertaining the index error at the 
two periods of greatest and least parallax, and so of other 
uncertainties. No error as to parallax arises from neglecting 
the unequal motion of the earth in its orbit, as far as regards 
the index error computed by the same stars at the two periods. 
But this selection of stars will be limiting the use of the in- 
strument, and the advantage of a mean of a number of ob- 
servations lost ; and in fact, with respect to the index errors 
used in determining the N. P. D. of a Lyrae, a Aquilae, and a. 
Cygni, as given by Mr. Pond, Phil. Trans, 1817, no parti- 
cular selection of stars, with a view to these points, seems to 
have been made. 
It may also perhaps be suggested, that the mural circle 
may be used without applying index error, as was done with 
