H M. Arago on Double Stars. 



When we look at the heavens with a telescope, even in those 

 quarters where the stars are most abundant, as in the milky way^ 

 the stars which are embraced in the fiL4d of vision are usually 

 distributed in a manner sufficiently uniform. The intervals 

 which separate them are nearly equal and vastly great. The 

 more this rule was general, the more the exceptions were likely 

 to attract the attention of astronomers. How, for example, was 

 it possible to avoid observing the star Castor (« Geminorum) 

 which to the naked eye appears single, and which, in truth, 

 had been so regarded by the Greek and Arabian astronomers, 

 but which is now found, when examined by a glass of sufficient 

 magnifying powers, to be composed of two stars of the third and 

 fourth magnitudes.'^ 



Among the double stars which are now known, there are 

 some, the component parts of which are exceedingly near to each 

 other. Before they can be observed or separated, it is neces- 

 sary to be supplied with first-rate glasses, of the largest dimen- 

 sions, and to be favoured with conditions of the atmosphere that 

 are but seldom found in these changeable climates. Of this 

 kind we may name s of Aries, y of the Crown, and tt of Her- 

 cules. 



The late Sir W. Herschel, who first bestowed continued atten- 

 tion on the double stars, has divided them into four classes, not 

 according to their intensity, but according to, the angular dis- 

 tance, greater or less, of the two component stars. The first 

 class includes all the groups in which the centres of the two stars 

 are at least four seconds of distance the one from the other. The 

 second class includes the angular distances between four and 

 eight seconds ; the third those between eight and sixteen ; and, 

 finally, i\\ejhurtli class is composed of all the groups which are 

 not comprehended in the preceding classes, and where the angu- 

 lar distance of the two stars does not exceed thirty- two seconds. 



Herschers first catalogue contained — 



97 double stars in class 1 



102 ....2 



114 3 



132 4 



Making a total of 445 double stars. 



Some time before his death, Herschel increased the number to 



