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XXV. On the parallax of α Aquilæ

The telescope erected for the investigation of the parallax of α Aquilæ, exactly resembles in its construction that which was employed for the observations of α Cygni. It has an achromatic object glass of ten feet focal length, and four inches diameter. I had at first selected β Canis minoris, as a proper star to be compared with α Aquilæ; 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 (l 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.