THE BOUNDARIES OF ASTRONOMY. 97 



origin of the tails of comets. I do not now inquire whether the re- 

 pulsion by which the tail is produced be due to the intense radiation 

 from the sun, or to electricity, or to some other agent. It is sufficient 

 for our present purpose to note that, even if the tails of comets do 

 gravitate toward the sun, the attraction is obscured by a more power- 

 ful repulsive force. 



The solar system is a very small object when viewed in comparison 

 with the dimensions of the sidereal system. The planets form a group 

 nestled up closely around the sun. This little group is separated from 

 its nearest visible neighbors in space by the most appalling distances. 

 A vessel in the middle of the Atlantic Ocean is not more completely 

 isolated from the shores of Europe and America than is our solar sys- 

 tem from the stars and other bodies which surround it in space. Our 

 knowledge of gravitation has been most entirely obtained from the 

 study of the bodies in the solar system. Let us inquire what can be 

 ascertained as to the existence of this law in other parts of the uni- 

 verse. Newton knew nothing of the existence of the law of gravita- 

 tation beyond the confines of the solar system. A little more is known 

 now. 



Our actual knowledge of the existence of gravitation in the celes- 

 tial spaces outside the solar system depends entirely upon those very 

 interesting objects known as binary stars. There are in the heavens 

 many cases of two stars occurring quite close together. A well-known 

 instance is presented in the star Epsilon Lyrae, where two stars are so 

 close together that it is a fair test of good vision to be able to separate 

 them. But there are many cases in which the two stars are so close 

 together that they can not be seen separately without the aid of a 

 telescope. We may take, for instance, the very celebrated double star 

 Castor, well known as one of the Twins. Viewed by the unaided eye, 

 the two stars look like a single star, but in a moderately good tele- 

 scope it is seen that the object is really two separate stars quite close 

 together. The question now comes as to whether the propinquity of 

 the two stars is apparent or real. It might be explained by the sup- 

 position that the two stars were indeed close together compared with 

 the distance by which they are separated ; or it could be equally ex- 

 plained by supposing that the two stars, though really far apart, yet 

 appeared so nearly in the same line of vision that when projected on 

 the surface of the heavens they seemed close together. It can not be 

 doubted that in the case of many of the double stars, especially those 

 in which the components appear tolerably distant, the propinquity is 

 only apparent, and arises from the two stars being near the same line 

 of vision. But it is, also, undoubtedly true that in the case of very 

 many of the double stars, especially among those belonging to the class 

 which includes Castor, the two stars are really at about the same dis- 

 tance from us, and, therefore, as compared with that distance, they are 

 really close together. 



VOL. XXIII. 7 



