s88 
Dr. Brinkley on the 
astronomy, the relative fitness of instruments for ascertaining 
with precision the smaller motions, whether real or apparent, 
of the fixed stars, is an object of importance. 
In instruments similar to that belonging to the Observatory 
of Trinity College, Dublin, the index error is found by 
reversing the instrument, the position of the vertical axis 
being ascertained by a plumb line. Thus the determination 
of the index error is not materially affected by any of the 
uncertainties above referred to. Therefore, by its principle, 
this instrument should appear particularly adapted for en- 
quiries relative to the annual parallax, annual variation, 
&c. &c. 
From the fixed telescopes we are probably to look for the 
final decision of the question of parallax. At first sight these 
seem to offer a very simple and certain criterion. However, 
a little consideration will point out probable sources of diffi- 
culty. Suppose the star under examination be compared with 
a star opposite in M, or with one as nearly so as can be con- 
veniently had. Besides the uncertainty respecting the annual 
variation, even the uncertainty in the quantity of aberration 
may tend in some degree to conceal the parallax, unless the 
minimum of aberration in declination of each star be at the 
same time, and the observations are made pretty equally on 
both sides of this time. The star (3 Aurigse has been judici- 
ously chosen by Mr. Pqnd to compare with a. Cygni. A 
more proper star could not have been chosen ; yet here the 
effect of an uncertainty in the maximum of aberration, 
amounting only to £ of a second, will have a sensible effect. 
If we suppose the maximum only 20", as I believe the 
maximum used by Mr. Pond is go"£, his winter distance fcr 
