34 THE VELOCITY OF LIGHT. 



Let us now suppose for a moment that the weight is dis- 

 connected from the drum, but that there is a handle at- 

 tached to the axis of it, by means of which we can turn the 

 wheel at any rate of velocity we choose. Let us also sup- 

 pose that the other part of the instrument that is, the 

 part that we placed on a distant hill, is so far away that 

 it would require one second for the light to pass over the 

 adjoining country to it, enter the tube, be reflected to the 

 end of it, and return. This would be impossible in fact, 

 since light travels at a rate which would carry it seven 

 times round the earth in that time. We can, however, sup- 

 pose it for the purpose of illustrating the action of the ap- 

 paratus. Let us suppose that we allow a flash to pass out 

 through a notch, and that an observer with his eye at A is 

 watching for its return. At the expiration of the second, 

 the time required for its journey, he would see it through 

 the glass, M, which, it will be remembered, was not silver- 

 ed, but only polished, so that, while it reflected a portion 

 of the light, it also allowed a portion to pass through. 



But if, on the other hand, the wheel were to be moved 

 while the ray of light was gone, so that, on the return, it 

 should find a tooth in its way to stop it, instead of an open- 

 ing to allow it to pass through, it is plain that the ye at 

 A would see nothing. And, moreover, if the wheel were 

 made to revolve regularly at a rate which should bring a 

 notch and a tooth alternately into the path of the ray at 

 intervals of a second, then every flash which went out 

 through the notch would find a tooth in its way to intei-- 

 cept it when it came in, and the eye at A would not see 

 the light at all. 



If it required less than a second, as, indeed, it actually 

 must, for the ray of light to pass to and fro, then all that 

 would be required to stop the flashes on their return would 

 be to make the wheel turn faster; and it is easy to see 



