By observing a star in both positions of the instrument, the zenith distance is obtained without reference to any other star. The Greenwich mural circle makes the polar distance of a star depend on the polar distance of several other stars. The polar distance of a star in the zenith, for instance, depends on the polar distances of stars many degrees from the zenith, so that, in fact, the place of a star in the zenith is affected by refraction, i. e. if we wish to compare the observed places of the star in winter and summer, we must do it by the intervention of stars affect ed by refraction. The stars used for comparison may not be the same stars, and then the places will be affected by the errors of the elements, such as nutation, &c. Indeed, it is quite obvious, that in this manner it will be extreme ly difficult to compare with exactness the places of a star at two opposite seasons. Had Bradley used such an in strument in the observation which led him to the disco very of the aberration of light, it is highly probable he would have been unable to disentangle the complicated results met with, and so he would have missed the disco very.
These remarks appear to show, that the mural circle is an instrument by no means adapted to the discovery of minute changes in the places of the stars. The vertical circle of the Dublin observatory is free from the above objections.
Accordingly, Dr. Brinkley early applied himself to ob serving certain stars with a reference to parallax. An ex tract of a letter from him to Dr. Maskelyne, giving an ac count of the parallax of a Lyra;, was published in the Philosophical Transactions for 1810.
In March, 1815, a paper was read before the Royal Irish Academy, in which Dr. Brinkley gives a minute account of his observations relative to this subject.
The stars in which Dr. Brinkley had observed a paral lax were, Arcturus, a Lyrae, a Aquilx, and a Cygni. His observations gave no parallax for the Pole Star and y Dra conis. We must refer to the above mentioned paper and its appendix, printed in the 12th vol. of the Transactions of the Royal Irish Academy, for the particulars and re marks of the author on these observations.
Mr. Pond having examined his observations, made with the mural circle in 1812,1813, and 1814, gives an account of the results in a paper printed in the Philosophical Transactions for 1817. Each of the stars in which Dr. Brinkley had appeared to find a sensible parallax, by Mr. Pond's observations, gave only a very small parallax, and this Mr. Pond considered to arise from some unknown cause not connected with parallax.
The unfitness, however, of the mural circle for these inquiries appears to have induced Mr. Pond to proceed
by another method. He procured two fixed telescopes to be erected at Greenwich, of considerable length. These were furnished with wire micrometers. One telescope was for observing a Cygni, and the other for a Aquilx. For comparison with a Cygni, R a star nearly op posite in right ascension, and having very nearly the same declination, was chosen. Both these stars passing through the field of view, the telescope remaining fixed, the difference of declination could be measured by the micrometer. The stars being nearly opposite in right as cension, the joint effects of parallax would be visible, or, in other words, the sum of the parallaxes would be observ ed instead of the difference.
Mr. Pond's observations are given in the volume of Philosophical Transactions for 1817, and appear to prove that the parallax of a Cygni does not amount to one-tenth of a second. But it is manifest, that, as the observations of the two stars were made at intervals of 12 hours, and often at intervals of 36 and 60, &c. hours, there was a danger of the telescope and its parts not remaining ex actly fixed, and there was no means of ascertaining this. Hence there might be a constant source of error to coun teract the effects of parallax. In fact, it appears that the telescope was subject to a motion arising apparently from the change of temperature. The instrument appears to have altered its position by several seconds between winter and summer ; and if we suppose that change of temperature between day and night caused the same effect in a smaller degree, the observations would not contradict a parallax of half a second, which is the result of a numerous ;cries of observations of this star made at Dublin.
It is a remarkable circumstance, that the change of place in the Greenwich fixed telescope, was such as might have been considered to indicate a considerable parallax in a Cygni. This is similar to what took place in Hooke's telescope; but Mr. Pond had guarded against such an er ror by using two stars, and so no derangement of the in strument could affect his results, excepting as far as it might take place between two succeeding observations.
In using the telescope for a Aquila, Mr. Pond appears not to have been able to avail himself of a star situate so as to obtain the sum of the parallaxes. The star fixed on by him 55 1 Pegasi, (Phil. Trans. 1818,) although differ ing three hours in right ascension, is circumstanced near ly the same as to the effects of parallax in declination, as if it had only differed by a few minutes in right ascen sion. Consequently, in this case, no advantage was de rived from using the fixed telescope.