ON LIGHT. 231 



periment (which is the simplest in its conception and 

 explanation), these glimpses are obtained by looking 

 through an opening in a screen corresponding exactly 

 in size and shape to one of the intervals between the 

 teeth of a metallic wheel which is made to revolve before 

 the opening, so that as the teeth pass in succession, they 

 intercept the light so long as they cover it j but allow it 

 to pass when, in place of a tooth, an interval is presented 

 before the eye. Imagine such a wheel, screen, and 

 opening, the wheel being at rest in the last-described 

 -ituation ; and through another such an opening in the 

 same screen, corresponding exactly in size, shape, and 

 situation to another of the intervals -between the teeth, 

 let a sunbeam be directed outwards, in a direction par- 

 allel to the axis of the wheel, by a highly-polished 

 reflector, so as to strike upon another such reflector so 

 placed at some considerable and measured distance from 

 the wheel, that the light shall be reflected back again by 

 this second mirror. By slightly inclining and properly 

 adjusting this it may be made to return, not to the orifice 

 from which it issued, but to the other behind which the 

 eye of the observer is placed. In this state of things, 

 when all is at rest, he will see the reflected light ; but if 

 the wheel be turned slowly round, a tooth will come be- 

 fore the first reflector in place of an opening, and inter- 

 cept the light then another opening, another tooth, and 

 so on, producing successive glimpses of light separated 

 by dark intervals. 



(14.) If the motion of the wheel be gradually accele- 

 rated, so that more than ten teeth pass before the orince 



