THE METHOD OF MEANS. . 415 



absolute coincidence, if it even occurs or seems to occur, 

 must be purely casual, and is no indication of precision. It 

 is one of the most embarrassing things we can meet when 

 experimental results agree too closely. Such coincidences 

 should raise our suspicion that the apparatus in use is in 

 some way restricted in its operation, so as not really to 

 give the true result at all a , or that the actual results have 

 not been faithfully recorded by the assistant' in charge of 

 the apparatus. 



If then we cannot get twice over exactly the same 

 result, the question arises, How can we ever attain the 

 truth or select the result which may be supposed to 

 approach most nearly to it 1 The quantity of a certain 

 phenomenon is expressed in several numbers which differ 

 from each other ; no more than one of them at the most 

 can be true, and it is more probable that they are all 

 false. It may be suggested, perhaps, that the observer 

 should select the one observation which he judged to be 

 the best made, and there will often doubtless be a feeling 

 that one or more results were satisfactory, and the others 

 less trustworthy. This seems to have been the course 

 adopted by some of the early astronomers. Flamsteed 

 when he had made several observations of a star probably 

 chose in an arbitrary manner that which seemed to him 

 nearest to the truth b . 



When Horrocks selects for his estimate of the sun's 

 semidiameter a mean between the results of Kepler and 

 Tycho he professes not to do it from any regard to the 

 idle adage, * Medio tutissimus ibis,' but because he 

 thought it from his own observations to be correct c . But 

 this method will not apply at all when the observer has 



Thomson and Tait, 'Treatise on Natural Philosophy,' vol. i. p. 309. 

 b Baily's ' Account of Flamsteed,' p. 376. 



c 'The Transit of Venus across the Sun,' by Horrocks, London, 1859, 

 p. 146. 



