210 Proceedings of the Royal Irish Academy. 



stood that in the present Paper these ohservations are not referred to^, 

 and that the Yakie of the parallax now presented has been solely 

 founded on the observations of the difference of declination between 

 (A) and the small star following. 



In adopting this course, I was also influenced by some other con- 

 siderations. In the researches of Strure on the annual parallax of 

 61 Cygni, he employed the star (B), as Dr. Briinnow had done. It 

 however appeared to me that on this very account a determination of 

 the parallax in which the preceding star was used would be, if suc- 

 cessful, of very considerable interest. As this was the first work of 

 the kind in which I had engaged, I was glad to have an opportunity 

 of the practice which it afforded, before I commenced a series of 

 measures which were to be amalgamated with those obtained by the 

 mature skiR of my predecessor. 



I have to regret that, owing principally to the exceedingly bad 

 weather which prevailed here during the winter of 1877-8, the num- 

 ber of observations is not so large as I would have wished. The ob- 

 servations yield thirty-six equations of condition for the determination 

 of the four unknown quantities on which the apparent difference of 

 declination depend. These equations contain the results of observa- 

 tions made on thirty-five different nights, which are only tolerably 

 "well distributed over the twelve months during which the observations 

 extend. The method of observing which I have used is almost iden- 

 tical with that employed by Dr. Briinnow in his researches on the paral- 

 lax of 61 Draconis {Ohservatiojis and Researches at Dunsink, Parts I. 

 and II.). A complete observation of the difference of declination is 

 the mean of eight independent determinations. 



The observations were reduced by employing the values of the 

 micrometer screws and the thermometric coefficients determined by 

 Dr. Briinnow (Part I., p. 8). The results were then cleared from 

 refraction by the application of the correction 



+ TiD cosec^ (8 + m), 



where D is the apparent difference of declination, S is the declina- 

 tion of 61 Cygni, and m is the function of the hour angle defined in 

 Bessel's Table, Astr. Tint. Bd. 1, p. 190, and computed here for the- 

 latitude of Dunsink (53° 23' 13"). 



To clear the observed difference of declination from the effects of 

 precession, aberration, and nutation, and reduce the result to the 

 date 1878-0, the following correction is applied: — ■ 



-}- 0"-05266 (1878 - t) 



+ [6-3089] i 



- [6-4137] h cos {E:+ a) 



+ [7-5735]^ sin (G^ + a) 



+ [7-3643] A sin (^+ a). 



