248 
Dr. Brinkley’s observations , &c. 
It appears to me, that the only method by which an expla- 
nation of the difficulties that have occurred, from a compa- 
rison of the Greenwich observations and of those made at 
this Observatory, can be obtained, is from an extensive series 
of observations of many stars, referring each to the apparent 
zenith point. I am therefore pursuing such a course of ob- 
servations. Conclusions as to the existence or non-existence 
of parallax, from comparisons of the relative places of stars 
taken indiscriminately, must be liable to much uncertainty, 
whether the comparisons be made by polar distances or by 
right ascensions. The former being affected by the uncer- 
tainty of refraction, may, at first view, be thought more sub- 
ject to error than the latter ; but a careful consideration of 
the circumstances attending the latter method, will show that 
it has its peculiar difficulties.* 
* As Mr. Bessel’s determination of the maximum of aberration has been referred 
to, it may also be right to mention his results respecting the parallax of certain 
stars. He uses transit observations of stars nearly opposite in right ascension (p. 1 10. 
&c.) Thus he finds the sum of the parallaxes of Sirius and a Lyras insensible, and 
the sum of the semi-parallaxes of Procyon and a Aquilae, nearly 1". This method of 
using the transit observations is undoubtedly far preferable to that of using them 
indiscriminately. With respect to the observations Mr. Bessel had to compute 
from, I think it must be allowed they were not sufficiently exact, to give much weight 
to his conclusions. The methods of observing with the transit, and of entering the 
observations, were then far inferior to the present. This objection, however, does 
not apply to the observations of the pole star, and therefore does not affect the maxi- 
mum of aberration deduced from the observed right ascension of that star. 
Observatory , Trinity College, Dublin, February 13 , 1819. 
