360 Proceedings of the Royal Irish Academy. 



0"'4654, found by the former series, is very satisfactory. The accor- 

 dance is indeed so close that there can be no doubt it is to some 

 extent due to the chapter of accidents. These investigations confirm 

 the supposition that the annual parallax of 61 Cygni is nearer the haK 

 second found by Struve than the third of a second found by Bessel. 



In order to shoTv the degree of accuracy which can be expected in 

 such observations, I give a diagram (Fig. 2) which represents the pre- 

 sent series of observations on the assumption of a parallax of 0"-4676. 

 The effect of such a parallax on the difference of declinations cannot 

 exceed 0"'40. The dots denote the observations, and the curve gives 

 the calculated effect of parallax. 



Though some of the discrepancies seem large, relatively to the 

 total amount to be measured, yet the greatest divergence of the obser- 

 vation from the curve is not more than the angle subtended by a penny- 

 piece at the distance of fifteen or twenty miles. 



(P m 242.) 



In the Monthly Notices, vol. xx. p. 8, is a Paper by 0. Struve, 

 " On a star which would be suitable for a parallax series." The star 

 in question is P m 242, its position for 1879-0 being a = 3^ 59™ 30% 

 and 8 = 37° 45''3. Argelander, in his Catalogue of 560 stars, had 

 pointed out that P iii 242 probably formed a wide Binary system 

 with the next following star of his Catalogue 50 Persei, on account 

 of the equality in direction and quantity of their large proper motions. 



Struve remarks also that P ui 242 is a double star of Herschel's 

 first class (Xo. 531 of 0. Struve's Catalogue), the components being of 

 6-7* and 8-9* magnitudes respectively, and 3" or 4" distant. The 

 physical correction of the two components of P m 242 appears to be 

 established by their equal proper motions. A fourth star (BD + 37°, 

 877, mag. 7-8) is south, preceding P in 242 at a distance of nearly 4'. 

 This star does not belong to the system formed by P m 242 and 50 

 Persei, for the changes in its position with respect to P in 242 cor- 

 respond exactly to the proper motion of the latter. " This star, there- 

 fore" (says Struve, in conclusion) " would be a very qualified object of 

 comparison for determining the relative parallax of P m 242, for 

 which a considerable amount is indicated by the proper motion, and by 

 the probable physical connexion with 50 Persei at 15' distance." 



Additional interest arises from Mr. Burnham's discovery that 

 BD + 37°, 877, is also a double star, the distance being 1". See 

 M. A. S., vol. xiiv. p. 158. 



So far as I know, no measures have hitherto been made with 

 the view of testing whether Struve's surmise as to the existence of a 

 parallax for P iii 242 could be substantiated. I therefore commenced 

 a series of observations in Jamxary, 1879, of the distance and position 

 of the comparison star which Struve suggested, from the larger star of 

 the pair P in 242. 



In an important feature, however, the observations now to be dis- 

 cussed are very different from those which had previously been made 



