98 THE POPULAR SCIENCE MONTHLY. 



Among the splendid achievements of Sir William Herschel, one of 

 the greatest was his discovery of the movements of the binary stars. 

 It was shown by Herschel that in some of the double stars one star of 

 the pair was moving around the other, and that their apparent dis- 

 tances were changing. The discoveries inaugurated by Herschel have 

 been widely extended by other astronomers. One of the more rapidly 

 moving of the double stars lies in the constellation of Coma Berenices- 

 The revolution of one component around the other requires a period 

 of 25'7 years. The two components of this star are exceedingly close 

 together, the greatest distance being about one second of arc. There 

 is very great difficulty in making accurate measurements of a double 

 star of which the components are so close. More reliance may con- 

 sequently be placed upon the determination of the orbits of other bi- 

 nary stars of which the components are farther apart. Among these 

 we may mention the remarkable binary star | Ursas Majoris. The dis- 

 tance between the two components of this star varies from one second 

 of arc to three seconds. The first recorded measurement of this ob- 

 ject was by Sir William Herschel, in 1781, and since that date it has 

 been repeatedly observed. From a comparison of all the measure- 

 ments which have been made it appears that the periodic time of the 

 revolution of one of these components about the other is about sixty 

 years. This star has thus been followed through more than one entire 

 revolution. The importance of these discoveries became manifest 

 when an attempt was made to explain the movements. It was soon 

 shown that the movements of the stars were such as could be explained 

 if the two stars attracted each other in conformity with the law of 

 gravitation. It would, however, be hardly correct to assert that the 

 discovery of the binary stars proved that the two stars attracted each 

 other with a force which varies inversely as the square of their dis- 

 tance. Even under the most favorable circumstances the observations 

 are very difficult ; they can not be made with the same accuracy as is 

 attained in observing the movements of the planets ; they have not 

 even the value which antiquity will often confer on an observation 

 which has not much else in its favor. There are probably many dif- 

 ferent suppositions which would explain all that has yet been observed 

 as to the motions of the binary stars. Gravitation is but one of those 

 suppositions. Gravitation will no doubt carry with it the prestige 

 acquired by its success in explaining phenomena in the solar system. 

 I do not know that any one has ever seriously put forward any other 

 explanation except gravitation to account for the movements of the 

 binary stars, nor is any one likely to do so while gravitation can con- 

 tinue to render an account of the observed facts ; but all this is very 

 different from saying that the discovery of the binary stars has $)roved 

 that the law of gravitation extends to the stellar regions. 



Except for what the binary stars tell us, we would know nothing as 

 to the existence or the non-existence of the law of gravitation beyond 



