Mr. Pond, in the same volume of the Transactions, has given the result of observations on the right ascen sion of a Aquila, but the objection here is, that the stars with which a. AquiIx was principally compared were those, which, according to the Dublin observations, appear to exhibit sensible parallaxes.
The most certain results, by right ascension, are to be expected, by using stars twelve hours asunder, by which the sum of the parallaxes in right ascension would be ob tained, as Roemer first attempted ; and, in like manner, as the sum of the parallaxes in declination arc obtained by the fixed telescopes. But here we have to depend on the permanency of position of the instrument for twelve hours, which is always to be distrusted.
Dr. Brinkley, in a paper in the Phil. Trans. for 1818, gives some additional results, and enumerates particularly the inconveniencies and sources of inaccuracy/that may •take place from mural circles. In the Phil. Trans. for 1821. the same astronomer has given the results of nu merous sets of observations made with the Dublin circle, commencing in 1818, for the purpose of examining the question in all its bearings. The number of observations amounted together to nearly 4000.
Among them are the results of observations of thir teen stars, for which both the parallax and constant of aberration are investigated.
The quantities are given in the following table.
The column containing the constants of aberration may be thought to possess considerable interest. It contains the results of probably the only direct observations that have been made, to investigate the constants of aberra tion, since Bradley made his observations. This column, and that for the parallax, were obtained from the observa tions of the respective stars, by the method of making the sums of the squares of the errors a minimum.
The small negative parallaxes that appear, must be at tributed to the unavoidable errors of observation. Had there appeared among these any negative parallaxes, amounting to nearly a second, it would have produced a powerful argument against a visible parallax.
Dr. Brinkley also observed several bright stars between 4 II and 10 H. AR. viz. Aldebaran, p Tauri, a Orionis, Procyon, &c. In these stars the effect of parallax in de
clination is only a small part of the whole. Therefore, had any of these stars shown a considerable parallax in declination, a decisive adverse argument would have been obtained. There were nine of these stars, and in all three hundred and forty-three summer observations were com pared with four hundred and fourteen made in winter. In no star was found a difference between the observations at the two seasons, greater than what might be attributed to the unavoidable errors of observation.
These stars are opposite in right ascension to those in which he had found a parallax, from which circumstance a decisive argument is deduced, that the changes which appeared, and which he attributed to parallax, were not from change of figure in the instrument, or from change of temperature, because the same arches nearly were used in both sets of stars. This argument is carried somewhat farther by the observations of the 'pole star. By the reversing property of this instrument, the same arches very nearly are used for the pole star, above the pole, as for Arcturus; therefore, if the appearances of parallax in Arcturus were derived from the instrument, it must take place in the pole star observed at the same seasons, and it does not.
For some other remarks of this nature, made by Dr. Brinkley, we must refer to the paper itself.
Notwithstanding the additional evidence which Dr. Brinkley conceives he has obtained by these numerous observations, he states other conclusions obtained by hint that may be thought to increase the difficulty. He insti tuted a series of observations on stars in the same part of the heavens as those in which he had found the appear ances of parallax. Of seventeen stars observed for this purpose, he only found two, 7 Draconis and a Aquarii that appeared to be not affected by parallax. This circum stance will, Dr. Brinkley observes, justly, perhaps, with many, add to the difficulty of admitting his explanation. They will be unwilling to admit that many of the smaller stars are nearer to us than many of the brighter. That in a certain part of the heavens, of considerable extent, many of the stars exhibit a sensible parallax.