THE FUTURE OF ASTRONOMY 113 



small that a long series of years was required and, in general, for want 

 of early observations of the precise positions of the faint stars, this work 

 was confined mainly to the bright stars. Photography is yearly adding 

 a vast amount of material available for this study, but the minuteness 

 of the quantities to be measured renders an accurate determination of 

 their laws very difficult. Moreover, we can thus only determine the 

 motions at right angles to the line of sight, the motion towards us or 

 from us being entirely insensible in this way. Then came the discovery 

 of the change in the spectrum when a body was in motion, but still this 

 change was so small that visual observations of it proved of but little 

 value. Attaching a carefully constructed spectroscope to one of the 

 great telescopes of the world, photographing the spectrum of a star, 

 and measuring it with the greatest care, provided a tool of wonderful 

 efficiency. The motion, which sometimes amounts to several hundreds 

 of miles a second could thus be measured to within a fraction of a mile. 

 The discovery that the motion was variable, owing to the star's revolv- 

 ing around a great dark planet sometimes larger than the star, added 

 greatly not only to the interest of these researches, but also to the labor 

 involved. Instead of a single measure for each star, in the case of the 

 so-called spectroscopic binaries, we must make enough measures to de- 

 termine the dimensions of the orbit, its form and the period of 

 revolution. 



What has been said of the motions of the stars applies also, in gen- 

 eral, to the determination of their distances. A vast amount of labor 

 has been expended on this problem. When at length the distance of a 

 single star was finally determined, the quantity to be measured was so 

 small as to be nearly concealed by the unavoidable errors of measure- 

 ment. The parallax, or one half of the change in the apparent position 

 of the stars as the earth moves around the sun, has its largest value for 

 the nearest stars. No case has yet been found in which this quantity is 

 as large as a foot rule seen at a distance of fifty miles, and for com- 

 paratively few stars is it certainly appreciable. An extraordinary degree 

 of precision has been attained in recent measures of this quantity, but 

 for a really satisfactory solution of this problem, we must probably 

 devise some new method, like the use of the spectroscope for deter- 

 mining motions. Two or three illustrations of the kind of methods 

 which might be used to solve this problem may be of interest. There are 

 certain indications of the presence of a selective absorbing medium in 

 space. That is, a medium like red glass, for instance, which would cut 

 off the blue light more than the red light. Such a medium would 

 render the blue end of the spectrum of a distant star much fainter, as 

 compared with the red end, than in the case of a near star. A measure 

 of the relative intensity of the two rays would serve to measure the dis- 

 tance, or thickness of the absorbing medium. The effect would be the 



