PROPAGATION OF LIGHT. 7 



wheel. But if, on the other hand, the wheel be made to re- 

 volve rapidly, its velocity may be such that the light trans- 

 mitted through the opening at one extremity of the diameter 

 shall not pass through the opposite aperture on its return, but 

 be intercepted by the adjacent tooth ; and it will be conti- 

 nually invisible to the eye, so long as the wheel revolves with 

 the same velocity. If the velocity of the wheel be doubled, 

 the light will be transmitted, on its return, through the suc- 

 ceeding opening, and will reappear to the eye. If the velo- 

 city be trebled, the light will be intercepted by the next tooth, 

 and there will be a second eclipse ; and so on. 



It is plain that if the velocity of the wheel, correspond- 

 ing to the 1st, 2nd, 3rd, or m th eclipse, be known, the ve- 

 locity of the light may be calculated. Thus, if the wheel 

 makes n revolutions in a second, and has p teeth, the time 



of passage of one tooth across the same point of space = - 

 of a second. Consequently, the first eclipse will correspond 



to - of a second. But in the same time the light has twice 

 2np 



traversed the distance between the wheel and the mirror. If, 

 therefore, that distance be denoted by a, the velocity of light 

 will be 



V=2a*2np. 



If n be the number of revolutions in a second correspond- 

 ing to the m th eclipse, the velocity of light will be given by 

 the formula, 



2np 



V=2a 



2m -1' 



The apparatus devised by M. Fizeau for this experiment 

 is ingenious and effective. It consists of two telescopes, di- 

 rected towards each other, and so adjusted that an image of 

 the object-glass of each is formed in the focus of the other. 



