Ball— P«/r///«^ 0/ 61 {A) Cy(jnL 209 



TXXTY. — Eeseaeches on the Paealeax oe 61 (A) Cygjji, made at 

 Dtjnsink. By Robert S. Ball, LL. D., F. E,. S., Andrews Pro- 

 fessor of Astronomy in the University of Dublin, and Eoyal 

 Astronomer of Ireland. 



[Eead November 30, 1878.] 



On tte 22nd of August, 1868, my predecessor. Dr. Briinnow, com- 

 menced a series of observations with the South Equatorial, at Dunsink 

 Observatory, with the view of making a new determination of the 

 annual parallax of 61 Cygni. The method he adopted was to observe 

 the diiference of declination between the following star (B) of 61 Cygni 

 and a star of the 9-10 magnitude, which in 1868 followed in 51*5 sees. 

 at a distance of 104" to the north. These observations were repeated 

 occasionally by Dr. Briinnow, and the last was made on the 24th 

 of May, 1873. There are altogether twenty determinations of the 

 difference of declination between the two dates I have mentioned. 



After I had had some practice in the use of the South Equatorial, 

 I recommenced the series of observations on the differences of decli- 

 nation between 61 Cygni and the following star, which Dr. Briinnow 

 employed. But there is an important difference between my series 

 of observations, as here recorded, and those made by Dr. Briinnow, to 

 which I have already referred. In the latter case the folJoiving com- 

 ponent (B) of 61 Cygni was used, while in my observations it is the 

 difference in declination between the 'preceding component (A) and the 

 small star following which has been observed. It will naturally be 

 asked why I did not lase the same component as Dr. Briinnow had 

 done, and thus render the two sets of observations capable of being- 

 incorporated together. I confess that in the first instance this was 

 due to an inadvertence on my part. I was at first under the impres- 

 sion that Dr. Briinnow had used the preceding star (A) of 61 Cygni, 

 and under this impression I commenced my work by measuring the 

 difference of declination between (A) and the small star following. 

 It was not until I had made a large number of observations that I 

 became aware the component I was using was not the same as that 

 which Dr. Briinnow employed ; and when this discovery was made, 

 the question arose as to the best course to be adopted. I felt re- 

 luctant to discard the work I had already done, and recommence anew 

 with the other component, and therefore I resolved to complete the 

 series of measurements which I had commenced, and thus, in the first 

 instance, to seek for a determination of the annual parallax from my 

 own observations alone. At the same time, I decided to commence as 

 soon as possible another series of observations which would be strictly 

 in continuation with Dr. Briinnow's work. At the present date these 

 observations have made considerable progress, but their reduction or 

 discussion has not yet been commenced. It will therefore be under- 



