232 ON LIGHT. 



in a second of time, these glimpses run together into 

 continuous vision ; and if considerably more numerous 

 (suppose fifty or sixty per second), the light is perceived 

 steadily as if the wheel were at perfect rest only, how- 

 ever (if the intervals between the teeth be exactly equal 

 to the breadths of the latter), of half the brilliancy, see- 

 ing that only half the quantity of light will have entered 

 the eye in the same time. The motion of the wheel 

 still continuing to be accelerated, however, when it has 

 attained a certain very great rapidity the light is gradu- 

 ally perceived to grow feebler and at length altogether 

 disappears. This happens when the velocity of rotation 

 is such as to bring a tooth of the wheel precisely to 

 cover the whole of the orifice in the screen into which 

 the returning beam should be delivered at the very 

 moment of its arrival, so closing it up altogether that 

 is to say, when the rotation is just so rapid as to carry 

 each tooth over its own breadth during the time taken 

 by the light to go and return. When this happens, 

 suppose the acceleration of the wheel to cease, and its 

 motion to be maintained uniform. Then by counting 

 the turns made per minute by the driving-handle of the 

 train of wheel-work, or otherwise registering its speed ; 

 and knowing (from the construction of the train) how 

 many turns of the wheel correspond to one of the driver, 

 as also how many teeth it carries, the exact duration of 

 this interval, no matter how minute, can be exactly 

 computed, so that the time and the space run over by 

 the light in that time both become known. 



(15.) If the rotation be now still further accelerated, 



