230 THE POPULAR SCIENCE MONTHLY. 



of so singular a community of motion), the mind is lost in contemplat- 

 ing the immensity of the periods which the revolutions of the compo- 

 nents of the system must occupy. Madler had already assigned to the 

 revolution of Alcor around Mizar (Zeta Ursse) a period of more than 

 7,000 years. But, if these-stars, which appear so close to the naked eye, 

 have a period of such length, what must be the cyclic periods of stars 

 which cover a range of several degrees upon the heavens?" (From 

 Zeta to Beta is a distance on the heavens of about 19.) "The pecu- 

 liarities of the apparent proper motions of the stars," I added, " lend a 

 new interest to the researches which Dr. Huggins is preparing to make 

 into the stellar proper motions of recess and approach." 



But a few months later, in a lecture delivered at the Royal Institu- 

 tion, I pointed out more definitely what result I expected from Dr. Hug- 

 gins's researches. " Before long," I said, " it is likely that the theory 

 of star-drift will be subjected to a crucial test, since spectroscopic 

 analysis afibrds the means of determining the stellar motions of recess 

 and approach. The task is a very difficult one, but astronomers have 

 full confidence that in the able hands of Dr. Huggins it will be suc- 

 cessfully accomplished. I await the result with full confidence that it 

 will confirm my views." 



It will be manifest that if the five large stars in Ursa are really 

 travelling in the same direction, then, when Dr. Huggins applied the 

 new method of research, he would find that, so far as motion in the 

 line of sight was concerned, these stars were either all receding or all 

 approaching at the same rate, or else that they were all alike in show- 

 ing no signs of any motion, either of recess 01 approach. 



But in the mean time there was another kind of evidence which the 

 spectroscope might give, and on which I formed some expectations. 

 If these stars form a single system, it seemed likely that they would all 

 be found to be constituted alike in other words, that their spectra 

 would be similar. Not, indeed, that associated stars always display 

 such similarity. Indeed, the primary star of a binary system not un- 

 frequently exhibits a spectrum unlike that of the small companion. 

 But the five large stars in Ursa, being obviously primary members of 

 the scheme they form, might be expected to resemble each other in 

 general constitution. Moreover, since the stars not included in the set 

 viz., Alpha and Eta might be regarded as probably very much 

 nearer or very much farther away, it was to be expected (though not 

 so confidently) that these two stars would have spectra unlike the 

 spectrum common (on the supposition) to the five stars. 



Now, Secchi announced that the stars of the Great Bear, with the 

 exception of Alpha, have spectra belonging to the same type as the 

 spectrum of the bright stars Sirius, Vega, Altair, Regulus, and Rigel. 

 This result was in very pleasing accordance with the anticipations I 

 had formed, except that I should rather have expected to find that the 

 star Eta had a spectrum unlike that of the remaining five stars of the 



