OF ELECTRICITY AND THE DURATION OF ELECTRIC LIGHT. 585 



By screwing the axis of the mirror to a machine with multiplying wheels, I was 

 enabled to cause it to revolve fifty times in a second. The reflected image of a lumi- 

 nous point, therefore, passed over half a degree in the 72,000dth part of a second, 

 the angular velocity of the image being, as before noticed, double that of the mirror. 

 An arc of half a degree is easily estimated by the eye, and is equal to about an inch 

 seen at the distance of ten feet. Supposing this to be the limit of distinct observa- 

 tion, though perhaps a much smaller arc might be distinguished even by the unas- 

 sisted eye, we might expect, when a line of electric light is placed parallel to the axis 

 of the revolving mirror, to ascertain two things : first, the duration of the light at 

 each point where it appears ; and secondly, the time which elapses between the ap- 

 pearance of the light in two successive points of its path ; provided that the time, in 

 either case, be not less than the 72,000dth part of a second. The first would be in- 

 dicated by the horizontal elongation of the reflected image, and the second by the 

 distance between two lines drawn from the images perpendicular to the horizontal 

 plane. If the duration and velocity were both rendered sensible by the mirror, the 

 reflected image would appear as a deflected band of light. 



I successively presented to the mirror, sparks four inches in length drawn from the 

 prime conductor of a powerful electrical machine ; the explosions of a charged jar ; 

 a glass tube four feet in length, exhibiting a spiral of electric sparks passing between 

 dots of tinfoil ; an exhausted glass tube six feet in length, through which the spark 

 passed, and produced an unbroken line of attenuated electric light ; various pictures, 

 such as birds, stars, &c., formed of electric sparks. But in all these cases, when the 

 reflected images occurred within the field of view, they appeared perfectly unaltered, 

 and precisely as they would have done had they been reflected from the mirror while 

 at rest. 



When sparks were made to follow each other quickly, several reflected images 

 were simultaneously seen in different positions, owing to the images having been 

 renewed before the visual impression caused by the first had disappeared. The ex- 

 hausted tube being held near a prime conductor, when looked at directly, will some- 

 times appear to gleam with a continuous light ; but examined in the mirror, this 

 apparent continuity is seen to be owing to a rapid succession of transient flashes. 



For some experiments another position of the revolving mirror is preferable to that 

 just described. Fig. 5 represents the reflecting surface inclined to the axis of rotation, 

 and nearly perpendicular to it. If a luminous point be placed anywhere in the prolon- 

 gation of the axis, its images, successively reflected from diff'erent parts of the mirror, 

 form together a circle, the whole circumference of which may be seen at once. In 

 this form of the experiment the angular velocity of the image is equal to that of the 

 mirror, and both move in the same direction ; whereas in the former case the image 

 moved with double the velocity of the mirror, and in the opposite direction. The 



