98 Scientific Proceedings, Royal Dublin Society. 



where a large proportion of a revolution lias been accomplislied 

 under observation, in which case the latter part of the construction 

 {i. e. that for the axes) may with advantage be resorted to. 



In the case of A Cygni, which is illustrated in Plate IX., and in 

 which an arc of only about 45° of the orbit has been described 

 since the discovery of its duplicity, the points numbered from 1 to 10 

 in the figure, representing successive positions of the satellite in its 

 orbit, lie fairly well along the curve, which such a construction has 

 given as the apparent ellipse, and it is not easy to see how the ellipse 

 could be altered so as to suit them better. 



The centre Oi, and the axes OiAi and OiBi are those obtained as 

 in Plate YIII., from the five points 1,3, 5,7, and 9 ; and the centre 

 Oz, and the axes O2A2 and O^B^ are obtained from the points 2, 4, 

 6, 8, 10 ; while the centre 0, and the axes OA and OB are the 

 means of the former two, respectively, and are those with which the 

 ellipse has been constructed. 



