356 Proceedings of the Royal Irish Academy. 



4179. The result of a most elaborate series of raeasures made with 

 the heliometer gave for 61 Cygui a parallax of about one-third of a 

 second. 



Fifteen years later, Struve undertook a new determination of the 

 parallax of 61 Cygni, which had by that time moved into the position 

 marked 1853, Pig. 1. The comparison star employed by Struve was 

 of the 9-4 mag., BD + 38°, 4345. He employed measures of the dis- 

 tance and position angle of 61 (B) Cyg. fi'om the comparison star. 

 The result of Struve' s labours seemed to indicate that the parallax of 

 61 Cygni was haK a second. 



Considerable doubt existed as to which result was the correct one. 

 Auwers,^ on a discussion of the whole question, was inclined to agree 

 with Struve, and it was with the view of settHng the difference that 

 Dr. Brlinnow commenced the work. It had, however, been but little 

 more than commenced when Dr. Brunnow resigned his position at Dun- 

 sink, and on me, as his successor, devolved the task of continuing and 

 completing the work. A first instalment of the Observations, and a 

 discussion of the result, has been already given in Part III., Dunsink 

 Ohservations. 1 now give a further instalment, completing the obser- 

 vations made by myself. The final discussion of the entire series, 

 which amalgamates Dr. Briinnow's work and my own, must be post- 

 poned until the publication of Part Y. of JDunsinh Ohservations. 



Dr. Brunnow employed the star BD + 38°, 4351, 9*5 mag., for the 

 comparison. On 31st March, 1878, I found that this star followed 

 61 (A) Cygni in 5P-5 and 66"-5, north. Briinnow had ascertained 

 that the measurement of difference of declination could be made with 

 extreme accuracy by the South equatorial, and consequently it was the 

 method of differences of declination that he employed. The first series 

 of observations were made with 61 (A) Cygni, and from them I de- 

 duced a parallax of 0"'4654, practically agreeing with Struve's result. 

 See Dunsink Ohservations, Part III., pp. 16-40. 



In the series now under discussion I employed the following star 

 61 (B) Cygni, which had then moved into the position marked 

 1878 on the figure.. This series was commenced on 18th September, 

 1878, and concluded on 2nd October, 1879, 



On each night of observation it was usual to take four complete 

 measui'es of the difference of declination. Each complete measure was 

 derived fi'om four readings of the screws, followed by four after the 

 wires had been crossed. The final result adopted for each night was 

 the mean of the four complete measures. 



To illustrate the nature of these observations, and the method 

 employed in their reduction, I here give the series of measures taken 

 on the 16th November, 1878, a date which I have selected at random. 



^ Alhandlungen der Akademie zu Berlin, 1868. 



