xxv. J ACCOEDANCE OF THEORIES. 5 61 



number of seconds in 8 m . 13*. 3 we find the velocity of light 

 to be about 192,000 miles per second. 



Nearly the same result was obtained in what seems a 

 different manner. The aberration of light is the apparent 

 change in the direction of a ray of light owing to the com- 

 position of its motion with that of the earth's motion 

 round the sun. If we know the amount of aberration and 

 the mean velocity of the earth, we can estimate that of 

 light, which is thus found to be 191,100 miles per second. 

 Now this determination depends upon a new physical 

 quantity, that of aberration, which is ascertained by direct 

 observation of the stars, so that the close accordance of the 

 estimates of the velocity of light as thus arrived at by dif- 

 ferent methods might seem to leave little room for doubt, 

 the difference being less than one per cent. 



Nevertheless, experimentalists were not satisfied until 

 they had succeeded in measuring the velocity of light by 

 direct experiments performed upon the earth's surface. 

 Fizeau, by a rapidly revolving toothed wheel, estimated the 

 velocity at 195,920 miles per second. As this result dif- 

 fered by about one part in sixty from estimates previously 

 accepted, there was thought to be room for further investi- 

 gation. The revolving mirror, used by Wheatstone in 

 measuring the velocity of electricity, was now applied in a 

 more refined manner by Fizeau and by Foucault to deter- 

 mine the velocity of light. The latter physicist came to 

 the startling conclusion that the velocity was not really 

 more than 185,172 miles per second. No repetition of ths 

 experiment would shake this result, and there was accord- 

 ingly a discrepancy between the astronomical and the ex- 

 perimental results of about 7,000 miles per second. The 

 latest experiments, those of M. Cornu, only slightly raise 

 the estimate, giving 1 86, 660 miles per second. A little 

 consideration shows that both the astronomical determina- 

 tions involve the magnitude of the earth's orbit as one 

 datum, because our estimate of the earth's velocity in its 

 orbit depends upon our estimate of the sun's mean distance. 

 Accordingly as regards this quantity the two astronomical 

 results count only for one. Though the transit of Venus 

 had been considered to give the best data for the calcula- 

 tion of the sun's parallax, yet astronomers had not neglected 

 less favourable opportunities. Hansen, calculating from 



