56 Mr. Pond on the parallax of a. Lyrce. 
and a Lyras have the same parallax, or that their difference 
of parallax is zero ; but they have no tendency to show what 
is the actual magnitude of the parallax that the two stars 
have in common. If indeed we admit it to be proved, by 
the observations of Bradley, and the more recent ones of Dr. 
Brinkley, that the parallax of <y Draconis is insensible, we 
may then infer from the observed difference what is the 
parallax of the other star. But the method of investigation 
that we are now about to consider, does not depend on such 
an admission. 
Having successfully adopted the method of observing by 
reflection, I was desirous of employing it in a series of obser- 
vations upon u Lyras, with a view to determine this question. 
This series began on the ist of July, 1822, and has been 
continued to the present time.* Although this period em- 
braces only half the interval in which the greatest change or 
double parallax is affected, a circumstance which at first may 
appear very disadvantageous, yet that is more than compen- 
sated, in my opinion, by the number of observations, and by 
a uniformity of temperature, such as never can be expected 
in the extreme seasons of winter and summer. 
In observations of this nature the effects of temperature 
upon the instrument itself, and the uncertain refractions of 
the ray of light when brought into the lower part of the 
room, may produce errors of no inconsiderable magnitude, 
with reference to a question of so much nicety as the present. 
I can show however in the present as in the former process, 
that no error from temperature, affecting the instrument, has 
* Since the date of this paper, the observations have been continued throughout 
the winter, and the results will be found in the subjoined Table. 
