230 ON LIGHT. 



nomenon in question. The rain-drop is the light ; the 

 tube, a telescope; the screen at its lower end, a mi- 

 crometer; and the two opposite directions of the 

 observer's motion, the two tangents at opposite sides of 

 the earth's orbit at right angles to the situation of a star 

 as viewed from either. And the angle in question is 

 what astronomers call their " Constant of Aberration " 

 a very minute one indeed, but perfectly well measurable 

 amounting to about a third of a minute (20" -45), from 

 which it results that the velocity of light is about ten 

 thousand (more exactly 10,089) times that of the earth 

 in its orbit, which we know to be very nearly 19 miles 

 (i8'923) per second, which gives 190,860 miles per 

 second for the velocity of light. 



(13.) Two different experimental processes for measure- 

 ing this velocity have been devised and executed the 

 one by M. Fizeau, of the Parisian Academy of Sciences; 

 the other by M. Leon Foucault, recently and most de- 

 servedly elected into the same illustrious body ; the 

 inventor of that elegant instrument, the Gyroscope. 

 Both depend on the principle that the impression left on 

 the eye by any luminous object persists for a sensible, 

 though very minute, time (about the tenth of a second); 

 so that an object presented to the sight by successive 

 glimpses only, following each other more frequently than 

 ten times in a second, is seen continuously. If only 

 just so frequent, a fluttering is perceived ; but this 

 diminishes as the rapidity of presentation is increased : 

 and when much more frequent, distinct and perfectly 

 uninterrupted vision is produced. In M. Fizeau's ex- 



