286 THE PRINCIPLES OF SCIENCE. 



the double stars. It has since been estimated by Struve, 

 that the odds are 9*5 70 to i against any two stars of not 

 less than the seventh magnitude falling within the appa- 

 rent distance of four seconds of each other by chance, and 

 yet ninety-one such cases were known when the estimation 

 was made, and many more cases have since been discovered. 

 There were also four known triple stars, and yet the odds 

 against the appearance of any one such conjunction are 

 173,524 to 1 1 The conclusions of Michell have been en- 

 tirely verified by the discovery that many double stars are 

 in connexion under the law of gravitation. 



Michell also investigated the probability that the six 

 brightest stars in the Pleiades should have come 

 by accident into such striking proximity. Estimating 

 the number of stars of equal or greater brightness at 

 1500, he found the odds to be nearly 500,000 to i 

 against casual conjunction. Extending the same kind of 

 argument to other clusters, such as that of PraBsepe, the 

 nebula in the hilt of Perseus' sword, he says m : ' We 

 may with the highest probability conclude, the odds 

 against the contrary opinion being many million millions 

 to one, that the stars are really collected together in 

 clusters in some places, where they form a kind of system, 

 while in others there are either few or none of them, to 

 whatever cause this may be owing, whether to their 

 mutual gravitation, or to some other law or appointment 

 of the Creator/ 



The calculations of Michell have been called in question 

 by the late James D. Forbes n , and Mr. Todhunter vaguely 



I Herschel, 'Outlines of Astronomy,' 1849, p. 565; but Todhunter, 

 in his 'History of the Theory of Probability,' p. 335, states that the 

 calculations do not agree with those published by Struve. 



m 'Philosophical Transactions,' 176?, vol. Ivii. p. 431. 



II 'Philosophical Magazine,' 3rd Series, vol. xxxvii. p. 401, December, 

 1850; also August, 1849. 



