224 LECTURES TO SCIENCE TEACHERS. 



intimate knowledge of the nature of electrical and magnetic 

 phenomena. Consequently different people have under- 

 taken again to repeat the measurements of the velocity of 

 light. 



M. Cornu at Paris has repeated the experiment on 

 Fizeau's plan, but he avoided this little source of error 

 which I pointed out to you in making the determination of 

 the velocity of rotation. He has an electric contact with 

 one of these wheels, which marks upon a chronograph each 

 revolution of the wheel. The chronograph is also in con- 

 nection with a clock, which works a dot at each second ; 

 and by comparing the marks upon the strip of paper which 

 are impressed upon it by this toothed wheel, and the marks 

 impressed by the seconds pendulum of a clock, he is able 

 to tell at any instant the exact velocity of rotation of the 

 toothed wheel. He has also introduced probably other 

 great improvements into the method of Fizeau, but the 

 experiments have not been published in detail, so that we 

 can hardly judge at present of their absolute accuracy and 

 of their agreement with each other, but they have gone to 

 show that the result obtained by Foucault was extremely 

 close to what he found from his experiments by the method 

 of Fizeau. 



In conclusion, it may be worth while perhaps to point 

 out a slight adaptation of the method of Fizeau which was 

 undertaken by Mr. Young previous to the time when Cornu 

 published his results. It has not as yet been completed, 

 but at the same time it is perhaps worthy of being pointed 

 out. The difficulty which is attempted to be got over is 

 this : Fizeau had to observe the exact time when an eclipse 

 took place of the distant spot of light the exact velocity 

 which was required to give the eclipse. Now you all know 

 perfectly well that a very feeble light, when it becomes 

 nearly eclipsed, will be so feeble that it will be invisible to 

 the eye, so that there will be a sensible variation of 

 velocity, during which the light will always seem to be 

 eclipsed ; and in order to get any accurate value at all 

 M. Cornu in his experiment observed the velocity which 

 first gave an eclipse and the velocity when he first saw a 

 return of the light, and took the mean of those to indicate 

 the true velocity, which gave the absolute eclipse. But it 

 is very difficult to say when an eclipse actually takes place. 



