2-18 THE PRINCIPLES OF SCIENCE. [CHAP. 



entirely verified by the discovery that many double stars 

 are connected by gravitation. 



Michell also investigated the probability that the six 

 brightest stars in the Pleiades should have come by 

 accidents 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 Praesepe, the nebula in the 

 hilt of Perseus' sword, he says : l " 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, 2 and Mr. Todhu liter vaguely 

 countenances his objections, 3 otherwise I should not have 

 thought them of much weight. Certainly Laplace accepts 

 MichelTs views, 4 and if Michell be in error it is in the 

 methods of calculation, not in the general validity of his 

 reasoning and conclusions. 



Similar calculations might no doubt be applied to the 

 peculiar drifting motions which have been detected by 

 Mr. E. A. Proctor in some of the constellations. 6 The odds 

 are very greatly against any numerous group of stars mov- 

 ing together in any one direction by chance. On like 

 grounds, there can be no doubt that the sun has a con- 

 siderable proper motion because on the average the fixed 

 stars show a tendency to move apparently from one point 

 of the heavens towards that diametrically opposite. The 

 sun's motion in the contrary direction would explain this 

 tendency, otherwise we must believe that thousands of 

 stars accidentally agree in their direction of motion, or are 



1 Philosophical Transactions, 1767, voL Ivii. p. 431. 



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

 1850 ; also August 1849. 



3 History, &c., p. 334. 4 Essen Philosophique, p. 57. 

 Proceedings of the Royal Society; 20 January, 1870 ; Philosophical 



Magazine, 4th Series, voL xxxix. p. 381. 



