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XVII. On the alleged Evidence for a Physical Cotmexion 

 between Stars forming Binary or Multiple Groups, arisingfjom 

 their Proximity alone. By Prof. J. D. Fo^t&es,F.R.S. ^c. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, Melrose, July 13, 1849. 



IN conformity with usage and with the spirit of your Journal, 

 I may perhaps be permitted to suggest a doubt as to the 

 legitimacy of certain reasonings with respect to the evidence 

 for the physical co?inexio?i of binary or multiple stars arising 

 ii-oni the mere fact of their juxtaposition, as stated and applied 

 by some of the most eminent writers on sidereal astronomy. 

 I should probably have hesitated to oppose my solitary opinion 

 to that entertained by the eminent writers whom I am about 

 to quote, had I not found it to be entirely supported by the 

 eminent authority of two friends to whom I separately pro- 

 posed it. 



Nearly a century ago, Mitchell computed the chances to be 

 500,000 to 1 against the stars composing the group of the 

 Pleiades he\i\g fortuitously concentrated within the small ap- 

 parent space w hich they occupy : and he thence infers the 

 probability of a physical connexion between them. Struve 

 has pushed this consideration much further. In his classifi- 

 cation of double stars he has applied the same argument to 

 estimate the improbability of the occurrence of even single pairs 

 of stars in close proximity. He " calculates the odds at 9570 

 to 1 against any two stars from the 1st to the 7th magnitude 

 inclusive, falling (if fortuitously scattered) within 4" of each 

 other. Now the number of such binary calculations actually 

 observed at the date of this calculation was already 91, and 

 many more have been added to the list. Again, he calculates 

 that the odds against any such stars fortuitously scattered fall- 

 ing within 32" of a third, so as to constitute a triple star, is 

 not less than 173,524 to 1. Now, four such combinations occur 

 in the heavens." Sir John Herschel, from whose Outlines of 

 Astronomy I take this statement of Struve's results, adds, 

 " the conclusion of a physical connexion of some kind or other 

 is therefore unavoidable*." 



Now I confess my inability to attach any idea to what would 

 be the distribution of stars or of anything else, if " fortuitously 

 scattered," much more must I regard with doubt and hesita- 

 tion an attempt to assign a numerical value to the antecedent 

 probability of any given arrangement or grouping whatever. 



* Outlines of Astronomy, p. 564. If I recollect aright, the passage does 

 not occur in the edition in Lardner's Cyclopaedia. 



