42 Mr. Tomlinson on the Action of 



a stream of electricity instead of the discharge ; as also 

 by the rapid succession of sparks from a magnet, but in any 

 case when the flash of light is distinct and sudden, the 

 effect is complete. 



Soap bubbles, blown with hydrogen or the mixed gases, 

 and fired by means of a filament of cotton passed through 

 a small tube, and wetted with alcohol ; gunpowder, done 

 up in the form of a boy's cracker ; fulminate of mercury 

 struck on an anvil, may all be successfully employed. 



These experiments were performed in a darkened room, 

 not of necessity, but the results are best observed in this 

 manner. In Mr. Wheatstone's experiment, the presence of 

 light, either natural or artificial, does not interfere with its 

 success. 



The experiment may be made to succeed by the flame of 

 a lamp or candle. In order to effect this I employed a 

 disk of pasteboard, twelve or thirteen inches in diameter, 

 with a narrow slit cut out, extending from the centre nearly 

 to the circumference, and connected with a multiplying 

 arrangement. The light of the lamp was condensed by a 

 lens, and thrown upon the back of the slitted disk, and the 

 black and red disk placed in the front of the former, so 

 as to receive a flash of light from the lamp every time 

 the slitted disk performed one revolution. On causing 

 both disks to revolve, the black and red spaces were dis- 

 tinctly brought out, assuming, however, a curved form. 



But, perhaps, the most convenient method of producing 

 this phenomenon, is to stand behind the slitted disk, while 

 in front of it, at the distance of two or three feet, the radi- 

 ated disk is made to rotate. On rotating the slitted disk 

 the effect is very complete. The radii are, however, curved 

 either upwards or downwards, according as the eye of the 

 observer is above or below the axis of the disk, except the 

 radii which, for the time being, are vertical to the axis 

 above and below, and these are not curved. This effect 

 takes place when the disks are revolving in the same direc- 

 tion. The order will be inverted, if the disks move in 

 opposite directions ; a change also will take place in the 

 direction of the curvature of the radii, according to the 

 angle at which the eye is placed. 



This experiment is somewhat analogous to one by Dr. 



