216 LECTURES TO SCIENCE TEACHERS. 



round its circumference a number of notches, like the 

 teeth of a cog-wheel. This wheel is 4 inches in diameter, 

 and there are 1,000 notches, or teeth, in its circumference. 

 By the mechanism which you see appended, a great 

 velocity can be given to it. 



The principle upon which this acts is very simple in 

 theory, though it requires a little care in the practical 

 working. The object aimed at is to send a beam of light 

 past the edge of the disc, so as to pass through a notch 

 between two teeth. It is then to pass over a considerable 

 distance to a reflector, from which it is sent back. But on 

 reaching the revolving disc it may happen that the space 

 between two teeth through which the light passed before, 

 is now occupied by a tooth, so that the light is not allowed 

 to return, so that an observer will not see the reflected 

 light if the disc be rotating at such a rate as to bring a 

 tooth into the position previously occupied by a space, after 

 a time equal to the time taken by a light to pass over the 

 distance to the distant reflector and bask. It is thus clear 

 that if the experimental arrangements were properly 

 carried out, the observer would see no return of light. If, 

 however, the disc were rotated still more rapidly, it might 

 happen that in the time taken by light to pass over the 

 given distance the disc has revolved so that the return 

 light passes through the notch next to the one through 

 which it passed on its outward start. Thus, as we increase 

 the rate of rotation of the disc, we get alternately a full 

 return of light and complete darkness. 



Here we have the apparatus by means of which Fizeau 

 overcame the difficulties of the problem. The revolving 

 disc is placed in the focus of a telescope, the 'eye-piece is 

 detached, and between the disc and the eye piece there is 

 an inclined piece of glass. This serves as a reflector, to 

 send the rays from a light at the side along the axis of 

 the telescope, concentrating them on the edge of the 

 notched disc. From this point the rays diverge to the 

 object-glass of the telescope, whence they emerge parallel. 

 The apparatus is so arranged that the reflecting apparatus 

 is at a distance of some miles, and is seen in the telescope 

 just on the edge of the notched disc, which, of course, is 

 also in focus. Perhaps the most ingenious part of the 

 apparatus is the reflector, which may be described as a 



