5 o6 



HISTORY OF SCIENCE. 



distant it falls on the plane mirror D, by which it is reflected back to 

 B, and a portion traversing the glass B reaches the eye of the observer 

 at o', who, in fact, sees through a telescopic eye-piece the flame A' by 

 rays which have twice traversed the distance B D. The telescope at 



o is to enable the operators to 

 so adjust the apparatus at the 

 distant station that the ray re- 

 flected from B may return upon 

 its path. At c is seen edge- 

 ways a wheel with many teeth. 

 The teeth and the spaces be- 

 tween them are equal, and the 

 wheel can be maintained uni- 

 formly at any required rate of 

 revolution by clockwork not 

 shown in the figure. This 

 wheel occupies such a position 

 that the beam of light is inter- 

 cepted or allowed to pass ac- 

 cording as a tooth or a space 

 is in the axis of the tube. Sup- 

 pose that the wheel is in motion, 

 and that a space has permitted 

 a beam to pass out. If, during 

 the time that this beam takes 

 to reach D and return, the 

 wheel has moved so that a 

 tooth now occupies the centre, 

 the beam will be intercepted, 

 and the eye at o' would see no 

 light. If, then, the velocity of 

 the wheel c is gradually in- 

 creased until a complete eclipse 

 of the light takes place, this 

 state of things will be realized, 

 and from the velocity of the 

 wheel and the number of its 

 teeth it will be quite easy to 

 deduce the time required for 

 the light to pass to the distant 

 station and back. An increase of the wheel's velocity would cause the 

 light to come again into view, for the return ray would meet with a 

 second space in the place of the first. A further increase of the velocity 

 would again cause eclipse of the light, and that would give data for 

 another determination. A further increase of the velocity would again 

 produce an apparent extinction of the light, and so on. M. Fizeau's 



