C 477 3 
XXV. On the parallax of * Aquilce. By John Pond, F. R. S. 
Astronomer Royal. 
Read April 1 6, 1818. 
The telescope erected for the investigation of the parallax 
of a Aquilse, exactly resembles in its construction that which 
was employed for the observations of a, Cygni. It has an 
achromatic object glass of ten feet focal length, and four 
inches diameter. 
I had at first selected (3 Canis minoris, as a proper star to 
be compared with a Aquilas ; but I found upon trial that it 
could not be seen in the day time, except under such 
favourable circumstances as could seldom be expected. I 
considered this at the time as a great disappointment ; but 
I now find that the star (/ Pegasi) which I have substituted 
for it, is much better situated for the purpose. It has often 
been proposed to examine the parallax of a large star by 
comparing it with a smaller one ; but to do this by meridian 
observations is very difficult, and in most cases impossible, 
from some peculiar practical difficulties which I am about to 
state. 
For example : in the present case of a Aquilae, the smaller 
star, must have very nearly the same polar distance. If it 
follows the larger star within three hours of right ascension, 
it cannot be employed for this purpose, because in winter 
time it will pass the meridian before sun set ; should the 
smaller star differ four hours or more in right ascension from 
the larger, it will in the summer pass the meridian after 
