ACCORDANCE OF QUANTITATIVE THEORIES, <kc. 201 



the probability of an hypothesis merely by its accordance 

 with a few results only. Its general analogy and accord- 

 ance with other known laws of nature, and the fact that 

 it does not conflict with any other probable theories, must 

 be taken into account, as we shall see in the next book. 

 The requisite condition of a good hypothesis, that it must 

 admit of the deduction of facts verified in observation, 

 must be interpreted in the widest possible manner, as 

 including all ways in which there may be accordance or 

 discordance. 



All our attempts at reconciliation having failed, the 

 only conclusion we can come to is that some unknown 

 cause of a new character exists. If the measurements be 

 accurate and the theory probable, then there remains a 

 residual phenomenon, which, being devoid of theoretical 

 explanation, must be set down as a new empirical fact 

 worthy of deliberate investigation. As a matter of fact 

 these outstanding residual discrepancies have often been 

 found to involve new discoveries of the greatest im- 

 portance. 



Accordance of Measurements of Astronomical Distances. 



One of the most instructive instances which we could 

 meet, as regards the manner in which different measure- 

 ments confirm or check each other, is furnished by the 

 determination of the velocity of light, and the dimensions 

 of the planetary system. Roemer first discovered that 

 light requires time in travelling, by observing that the 

 eclipses of Jupiter's satellites, although they of course 

 occur at fixed moments of absolute time, are visible at 

 different moments in different parts of the earth's orbit, 

 according to the distance of the earth and Jupiter. The 

 time occupied by light in traversing the mean semi- 

 diameter of the earth's orbit is found to be about eight 

 minutes. The mean distance of the sun and earth was 



