222 LECTURES TO SCIENCE TEACHERS. 



The results he communicated to the Academy in 1862 were 

 stated by him to be remarkably accordant. He said that 

 the experimental values he obtained did not differ by more 

 than one hundredth part of the whole quantity to be 

 measured, and finally, as the result of his work, he stated 

 that the velocity of light was not what it had been sup- 

 posed to be, but decidedly less ; instead of being 308 

 million metres per second he found it was only 298 mil- 

 lions. He pointed out the very great importance of this 

 result, because employing the constant aberration that 

 quantity which Bradley investigated, as it had been deter- 

 mined by astronomers, as being an angle of 20 '45", which 

 was the value Struve gave to it he found the parallax of 

 the sun was no longer 8-57", but 8*86", and consequently 

 by this measurement of the velocity of light he diminished 

 the distance of the sun by 3,000,000 miles. This was a 

 very important result, especially at the time at which it 

 was announced. M. Hanson, the illustrious astronomer, 

 whose tables of the moon are better than any constructed 

 before, pointed out that the theory of the moon did not 

 agree with observation, on the assumption that the distance 

 of the sun is 95,000,000 of miles, and in order to make 

 theory agree with observation it was necessary to reduce 

 the distance between the sun and the earth to about 

 92,000,000 of miles. Now this was exactly what Foucault 

 showed was necessary from the theory of the velocity of 

 light. Other circumstances combined to render this true, 

 and Mr. Stone, then Chief Assistant at the Royal Obser- 

 vatory at Greenwich, pointed out that when we discussed 

 the observation of the transit of Yenus of the last century, 

 by interpreting the words of the observers in the true 

 manner, those observations also did not give 95 millions 

 of miles, as Encke had supposed, but gave a distance of 

 somewhere about 92,000,000. All these facts combined 

 together seem to make it nearly certain that that was the 

 real distance of the sun from the earth. This was the 

 reason that all the civilised nations of the world combined 

 to get as accurate a value as they could from the transit of 

 Venus which took place in 1874. 



A great deal of importance attends this exact determina- 

 tion of the velocity of light, not only in this way, but also 

 in other matters. In the system of observation which has 



