372 ASTRONOMY 



might contend that spectroscopic researches upon the physical con- 

 stitution of the sun and stars are as distinct from mechanical investi- 

 gations which are concerned with these bodies as units in motion, 

 irrespective of their internal structure, as the science of chemistry 

 is distinct from that of mechanics. A brief examination, however, 

 will show that the respective fields of the two departments of 

 astronomy are so interwoven that they cannot readily be divorced. 

 Through the application of a principle of spectral analysis the con- 

 clusions of the older astronomy are not only supplemented, but they 

 are logically strengthened in their standing as concepts having 

 objective reality. For example, the theory that the earth moves in 

 an orbit about the sun, quite recently in the world's history a mere 

 hypothesis, requiring a veritable mountain of demonstration before 

 mankind was really justified in its acceptance, receives support from 

 direct observations of the motion of the earth in its orbit made evi- 

 dent in the investigation of stellar spectra. It may be conceded that, 

 from the ordinary point of view, this added evidence was not actually 

 required. But, from the standppint of the philosopher, this new 

 evidence cannot be regarded as superfluous. Likewise, the measure- 

 ment of the motions of planets in the line of sight similarly confirms 

 the theory of the solar system. Measurement of stellar motions in 

 the line of sight, by means of the spectroscope, connects the field 

 of astrophysics indissolubly with the astronomy that deals with 

 thwart motions as seen upon the face of the sky. The two lines of 

 research offer concurrent testimony that is of extremely great 

 philosophic importance in arriving at conceptions regarding motion 

 in the stellar universe. But the logical connection of the two branches 

 of astronomical activity has a more profoundly philosophical basis. 

 The discovery of the law of gravitation introduced a virtually new 

 object of inquiry to the attention of astronomers. To what extent 

 can likeness be traced in physical laws and circumstances that gen- 

 erally prevail among the celestial bodies, including the earth as one 

 of them? The rotation of the sun on its axis, the revolution of satel- 

 lites about the planets, and a few other disconnected facts of this 

 kind, had already suggested thoughts as to such likeness. But the 

 discovery of gravitation gave an immense stimulus to this idea. 

 The mere fact that every particle of matter in the solar system at- 

 tracts every other swept away nearly the last vestige of speculations 

 which predicated an essentially peculiar difference between terrestrial 

 and non-terrestrial matter as to fundamental qualities. Matter in 

 Saturn was found to be heavy in the same sense that matter on the 

 earth is heavy. Not so long ago comets were generally supposed to 

 be emanations of some mysterious substance, celestial will-o'-the 

 wisps. That idea is now relegated to the limbo of forgotten things; 

 and the essential likeness of comets to other celestial bodies, in sub- 



