DRIFTING OF THE STARS. 227 



known to the spectroscopist from what he has already learned by ex- 

 amining the bright lines of hydrogen. Now, if Sirius were receding 

 very rapidly, the wave-length corresponding to one of these lines 

 would be lengthened ; it would correspond, in fact, to a part of the 

 spectrum nearer the red end, or the region of longer light-waves, and 

 thus the dark line would be shifted toward the red end of the spec- 

 trum ; whereas, on the contrary, if Sirius were very rapidly approach- 

 ing, the dark line would be shifted toward the violet end of the spec- 

 trum. All that would be necessary would be that the rate of approach 

 or recession should bear an appreciable proportion to the rate at which 

 light travels, or 185,000 miles per second. For, reverting to our cork- 

 thrower, it is clear that, if he travelled up-stream or down-sti'eam at a 

 rate exceedingly minute compared with the stream's rate of flow, it 

 would be impossible for the observer down-stream to be aware of the 

 cork-thrower's motion in either direction, unless, indeed, he had some 

 very exact means of measuring the interval between the successive 

 corks. 



Now, the spectrum of a star can be made longer or shorter, accord- 

 ing to the disjDersive power employed. The longer it is, the fainter its 

 light will be ; but, so long as the dark lines can be seen, the longer 

 the spectrum is, the greater is the shift due to steller recession or ap- 

 proach ; and, therefore, the more readily may such recession or ap- 

 proach be detected. But, with the instrument used by Dr. Huggins 

 four years ago, it was hopeless, save in the case of the brilliant Sirius 

 (giving more than five times as much light as any other star visible in 

 our northern heavens), to look for any displacement due to a lower 

 rate of recession than some hundred miles per second (little more than 

 the two-thousandth part of the velocity of light). What was to be 

 done, then, was to provide a much more powerful telescope, so that 

 the stellar spectra would bear a considerably greater degree of disper- 

 sion. With admirable promptitude, the Royal Society devoted a large 

 sum of money to the construction of such an instrument, to be lent to 

 Dr. Huggins for the prosecution of his researches into stellar motions 

 of approach and recession. This telescope, with an aperture of fif- 

 teen inches, and a light-gathering power somewhat exceeding that 

 usual with that aperture, was accordingly completed, and provided 

 with the necessary spectroscopic appliances. Many months have not 

 passed since all the arrangements were complete. 



In the mean time, I had arrived at certain inferences respecting the 

 proper motion of the stars, on which Dr. Huggins's researches by the 

 new method seemed likely to throw an important light. 



More than three years ago, I had expressed my conviction that, 

 whenever the recorded proper motions of the stars were subject to a 

 careful examination, they would confirm the theory I had enunciated, 

 that the stars are arranged in definite aggregations of various forms 

 star-groups, star-streams, star-reticulations, star-nodules, and so on. 



