no SCIENTIFIC APPARATUS. 



The velocity of light is not only a very interesting subject in 

 itself, as one of the great physical units to be determined, but is 

 also of the highest importance in connection with astronomy. 

 Various methods have been devised for determining it. Omitting, 

 as apart from the present subject, indirect means, mention must 

 be made of the method of the revolving mirror, as used by Fou- 

 cault, and that of the toothed wheel adopted by Fizeau. In the 

 former case, a beam of light reflected from a revolving mirror is 

 again reflected normally by a fixed concave mirror. During the 

 time occupied by the passage of light from the first to the second 



mirror and back, the revolving mirror has undergone a slight 

 angular motion, which produces a small deflection of the beam. 

 From this deflection the velocity of light may be computed. In 

 the second method, a beam of light passes between the teeth of a 

 revolving toothed wheel, and it is then reflected back along the 

 same line from a mirror some miles distant. In general during 

 the revolution the observer will see the light so reflected as a 

 luminous point ; but at certain rates which form an arithmetical 

 series the light will be extinguished by an intervening tooth. 

 With a triple rate it will be again seen, and so on. From obser- 

 vations on the rates for which the light ceases to be visible the 



