DRIFTING OF THE STARS. 229 



Zeta (the middle horse), a star known to the Arabian astronomers as 

 the " Test," because to see this star was held a proof of good eyesight, 

 is moving in the same direction and at the same rate as Zeta and the 

 rest of this set. And besides this star (which has also been called 

 Jack by the middle horse), Zeta has a telescopic companion which also 

 accompanies him in his motion on the celestial sphere. 



After a careful consideration of these circumstances, and an analy- 

 sis of the probabilities in favor of and against the theory that the con- 

 currence of apparent motion was merely accidental, I came to the 

 conclusion that the five large stars and the two smaller ones form a 

 true drifting set. I found, on a moderate computation, that the odds 

 were upward of half a million to one against the concurrence being ac- 

 cidental ; and, since I had recognized other instances of concurrence 

 not less striking, I felt that it was morally certain that these stars be- 

 long to one star-family. 



The reader will perhaps not be surprised to learn, however, that 

 before publishing this conclusion I submitted it (in July, 1869) to one 

 who was, of all men, the best able to pronounce upon its significance 

 the late Sir John Herschel. I have the letter (dated August 1, 

 1869), which he sent in reply, before me as I write. The part relating 

 to my discovery runs as follows : " The considerations you adduce 

 relative to the proper motions of the stars are exceedingly curious and 

 interesting. Of late years catalogues have gone into much detail, and 

 w r ith such accuracy that these motions are of course much better known 

 to us than some twenty or thirty years ago. The community of proper 

 motion over large regions (of which you give a picture in Gemini and 

 Cancer) is most remarkable, and the coincidence of proper motion in 

 Beta, Gamma, Delta, Epsilon, and Zeta Ursae Majoris, most striking. 

 Your promised paper on this subject cannot fail to be highly inter- 

 esting." 1 



In a letter written on May 11, 1870, and referring not to another 

 letter of mine, but to my " Other "Worlds," Sir John Herschel remarked, 

 "The cases of star-drift such as that in Ursa Major are very striking, 

 and richly merit further careful examination." 



My first public expression of opinion respecting the star-drift in 

 Ursa Major was conveyed in the following terms : " If these five stars 

 indeed form a system (and I can see no other reasonable explanation 



1 He proceeds as follows (the passage is removed from the main text, as relating to a 

 different branch of the subject): "I cannot say that I am at all surprised at its being 

 found that the average proper motions of stars of small magnitudes are not less than those 

 of large, considering (as I have always done) that the range of individual magnitude (i. e., 

 lustre) must be so enormous that multitudes of very minute stars may in fact be our very 

 near neighbors." Compare my paper on " The Sun's Journey through Space," above re- 

 ferred to, which paper also deals with the point touched on in the next sentence of Sir John 

 Herschel's letter : " Your remark on the conclusion I have been led to draw, relative to 

 the small effect of the correction due to the sun's proper motion, will require to be very 

 carefully considered, and I shall of course give it every attention." 



