ON LIGHT. 23J 



the light begins to reappear, and gradually increases to 

 its former brightness^ in which state of things the ob- 

 structing tooth has been carried, in that same interval oi 

 time, quite clear of the opening, and tlie next notch 

 brought exactly opposite to it. With yet increased 

 speed, the light again vanishes, again reappears, and so 

 on alternately, as the second, third, or fourtli tooth or 

 notch is successively brought before the opening ; and 

 on comparing the velocities of rotation corresponding, 

 they are found to increase in arithmetical progression ; 

 which obviously ought to be the case. In W. Fizeau's 

 experiments,* the distance between the reflector and the 

 revolving wheel was about 8600 metres, thus giving for 

 the whole distance travelled over by the light going and 

 returning 17,200 metres, or about loj miles; and for 

 the time occupied in its journey, hardly more than the 

 1 8,000th part of a second. A velocity of 196,000 miles 

 per second was assigned by him as their final result, 

 exceeding by about one-sixtieth part that resulting from 

 the astronomical observations. 



(16.) The experiments of M. Foucault, however, 

 leave no doubt that this last result is too great. In 

 these experiments, instead of measuring these minute 

 intervals of time by the rotation of a toothed wheel, a 

 revolving reflector was employed in pursuance of an 

 idea suggested by Mr Wheatstone, and applied by him 



* The actual details of this experiment, as executed by M. 

 Fizeau, were somewhat more complicated. Telescopes were used, 

 &c. For clearness of explanation, we have reduced the whole 

 process to its simplest form of expression. 



