24 ANNUAL EECORD OF SCIENCE AND INDUSTRY. 



in a little more than two years, which is a very rapid mo- 

 tion. This star may turn out to be an extremely interesting 

 binary, and deserves the attention of all observers of double 

 stars. 



ON A CLASSIFICATION OF DOUBLE STARS. 



riammarion states that, in a general revision of double 

 stars, he has divided them into classes, which have nothing 

 in common with those of Herschel and Struve. These latter 

 founded their distinctions upon apparent relative distances 

 and magnitudes, whereas Flammarion has founded his classes 

 upon the physical peculiarities of the different systems. He 

 gives the name of stellar systems only to those of whicli 61 

 Cygni is the type, whose components, although physically 

 associated and animated by a considerable common proper 

 motion, do not revolve about each other, but move in straight 

 lines. The name oi perspective groups he gives to the double 

 stars whose relative motions are in straight lines, but whose 

 components are not physically associated, but are accident- 

 ally placed upon the same visual ray ; one of which stars, 

 therefore, is passing between us and the other by reason of 

 a difference in their proper motions. Such double stars are 

 far more numerous than the stellar systems, but less numer- 

 ous than those whose components have remained relatively 

 fixed since the time of their discovery. Of the stars be- 

 longing to the persj^ective group, an excellent example is 

 found in Yega or Alpha Lyroe. 6 B, LXXX., 662. 



ON THE DOUBLE STAK 61 CYGNI. 



The observations made by Bessel in his work on the 

 parallax of the star 61 Cygni led him to the conclusion 

 that this double star was moving with great velocity, and 

 that the two components were held together by the force 

 of attraction, and were describing orbits about a common 

 centre of gravity, the time of revolution being estimated 

 at about 400 years, and the annual parallax at less than 

 one half a second. The more recent observations of astron- 

 omers have, however, led Flammarion to conclude that no 

 orbit as yet computed for this star will satisfy the obser- 

 vations, and that we are forced to conclude that the small- 

 er of the two stars is moving with respect to the greater 



