202 THE PRINCIPLES OF SCIENCE. 



long assumed by astronomers as being about 95,274,000 

 miles, this result being deduced by Bessel from the ob- 

 servations of the transit of Venus, which occurred in 1769, 

 and which were found to give the solar parallax, or what 

 is the same thing, the apparent size of the earth as seen 

 from the sun, as equal to S"'5j8. Now, dividing the 

 mean distance of the sun and earth by the number of 

 seconds in 8 m . 1 3 s . 3 we find the velocity of light to be about 

 192,000 miles per second. 



Nearly the same result was obtained in an apparently 

 very different manner. The aberration of light is the 

 apparent change in the direction of a ray of light owing 

 to the composition of its motion with that of the earth's 

 motion round the sun. If we know the amount of aber- 

 ration and the mean velocity of the earth we can very 

 simply estimate that of light which is thus found to be 

 191,102 miles (166,072 geographical miles) per second. 

 Now this determination depends upon an entirely new 

 physical quantity, that of aberration, which is ascertained 

 by direct observation of the stars, so that the close accord- 

 ance of the estimates of the velocity of light as thus arrived 

 at by different 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 actually 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 differed by about one part in sixty from esti- 

 mates previously accepted, there was thought to be room 

 for further investigation. The revolving mirror, previously 

 used by Mr. Wheatstone in measuring the velocity of elec- 

 tricity, was now applied in a more refined manner by 

 Fizeau and by Foucault to determine the velocity of 

 light. The latter physicist finally came to the startling 



