50-i ADDITIONS. 



(as Nebulae, the Magnitude of Stars, and the like), the orbits of several 

 double stars are computed by the aid of the new observations. But 

 Sir John Herschel's conviction on the point in question, the operation 

 of -the Newtonian law of gravitation in the region of the stars, is ex- 

 pressed perhaps more clearly in another work which he published in 

 1849. 11 He there speaks of Double Stars, and especially of gamma. 

 Virginia, the one which has been most assiduously watched, and lias 

 offered phenomena of the greatest interest. 12 He then finds that the 

 two components of this star revolve round each other in a period of 

 182 years; and says that the elements of the calculated orbit repre- 

 sent the whole series of recorded observations, comprising an angular 

 movement of nearly nine-tenths of a complete circuit, both in anglf 

 and distance, with a degree of exactness fully equal to that of obser- 

 vation itself. "No doubt can therefore," he adds, "remain as to the 

 prevalance in this remote system of the Newtonian Law of Gravi- 

 tation." 



Yet M. Yvon de Villarceau has endeavored to show 13 that this con- 

 clusion, however probable, is not yet proved. He holds, even for the 

 Double Stars, which, have been most observed, the observations are 

 only equivalent to seven or eight really distinct data, and that seven 

 data are not sufficient to determine that an ellipse is described accord- 

 ing to the Newtonian law. Without going into the details of this 

 reasoning, I may remark, that the more rapid relative angular motion 

 of the components of a Double Star when they are more near each 

 other, proves, as is allowed on all hands, that they revolve under the 

 influence of a mutual attractive force, obeying ^the Keplerian Law of 

 Areas. But that, whether this force follows the law of the inverse 

 square or some other law, can hardly have been rigorously proved as 

 yet, we may easily conceive, when we recollect the manner in which 

 that law was proved for the Solar System. It was by means of an 

 error of eight minutes, observed by Tycho, that Kepler was enabled, as 

 he justly boasted, to reform the scheme of the Solar System, to show, 

 that is, that the planetary orbits are ellipses with the sun in the focus. 

 Now, the observations of Double Stars cannot pretend to such accu- 

 racy as this ; and therefore the Keplerian theorem cannot, as yet, have 

 been fully demonstrated from those observations. But when we know 



11 Outlines of Astronomy. & Out. 844. 



18 Connaissance des Temps, for 1852; published in 1849 



