464 FRAGMENTS OF SCIENCE 



there are other actions which far transcend in rapidity 

 that of the rifle-bullet. A flash of lightning cleaves a 

 cloud, appearing and disappearing in less than a hundred- 

 thousandth of a second, and the velocity of electricity is 

 such as would carry it in a single second over a distance 

 almost equal to that which separates the earth and moon. 

 It is well known that a luminous impression once made 

 upon the retina endures for about one-sixth of a second, 

 and that this is the reason why we see a continuous band 

 of light when a glowing coal is caused to pass rapidly 

 through the air. A body illuminated by an instantaneous 

 flash continues to be seen for the sixth of a second after 

 the flash has become extinct; and if the body thus illu- 

 minated be in motion, it appears at rest at the place 

 where the flash falls npon it. When a color-top with 

 differently-colored sectors is caused to spin rapidly the 

 colors blend together. Such a top, rotating in a dark 

 room and illuminated by an electric spark, appears mo- 

 tionless, each distinct color being clearly seen. Professor 

 Dove has found that a flash of lightning produces the 

 same effect. During a thunderstorm he put a color-top 

 in exceedingly rapid motion, and found that every flash 

 revealed the top as a motionless object with its colors dis- 

 tinct. If illuminated solely by a flash of lightning, the 

 motion of all bodies on the earth's surface would, as 

 Dove has remarked, appear suspended. A cannon-ball, 

 for example, would have its flight apparently arrested, 

 and would seem to hang motionless in space as long as 

 the luminous impression which revealed the ball remained 

 upon the eye. 



If, then, a rifle-bullet move with sufficient rapidity to 

 destroy life without the interposition of sensation, much 



