M. Arago on Double Stars. 3 



above 500. Since that time it has been very considerably in- 

 creased, M . Struve, after making a general examination of the 

 heavens with an immense telescope of Fraunhofer, after con- 

 ducting his investigations to all the stars of the first eight mag- 

 nitudes, and even to the most brilliant of those of the ninth, 

 which are comprehended between the north pole and 15" south 

 of the equator, has distinguished and catalogued (those of Her- 

 schel included), 



987 double stars of class 1 



675 2 



669 3 



736 4 



Making a total of 3057 double stars. 



This number of more than 3000 is the result of the exami- 

 nation of about 120,000 different stars; and thus we find that 

 about one star infirty may be considered as double. * 



Those observers who are favourably situated for the exami- 

 nation of the southern hemisphere, the astronomers of the Cape 

 of Good Hope and of Port Jackson, have also begun to direct 

 their attention to these multiple stars. Hence we may reason- 

 ably infer, that, in a very short time, the number of these stars 

 that will be subjected to an annual examination in the principal 

 observatories, will not be under 5000 or 6000. 



The division of double stars into four classes, proposed by 

 Herschel, and adopted by his successors, besides being com- 

 pletely arbitrary, has a defect which makes it necessary it should 

 be wholly abandoned. In fact, we shall presently see, that ac- 

 cording to the period of observation, we should be led to place 

 the same group, now in the first class, then again in the second, 

 and sometimes again in the third class. 



The triple and quadruple stars do tiot appear to be very nu- 



• This proportion, as M. Struve has observed, changes with the brightness 

 of the stars. Thus, of 2374 stars of the first to the sixth magnitude which 

 Flamsteed had examined in the region explored by the astronomer of Dorpat, 

 230 double ones were discovered ; that is somewhat less than 1 in 10. 



In the same region of the heavens, Piozzi has catalogued 3388 stars, which 

 the English astronomers had in part disregarded on account of their inferior 

 lustre. This group presents only 134 double stars; that is 1 in 25. 



In repeating the process on still wnaller stars, only I in 42 is found. 



A % 



