168 ELECTRIC-SPARK PHOTOGRAPHS. 



A knowledge of electrical principles enables one to modify the elec- 

 trical apparatus employed to make this spark in such a manner that its 

 duration may be greatly reduced without, at the same time, a very great 

 sacrifice of light ; but while this may be done it is important to be able to 

 observe how long the spark actually lasts when made by apparatus 

 altered little by little m the proper manner. The desired information 

 is at once given by the revolving mirror. For instance, every one is 

 aware how, by the turn of the wrist, one may reflect a beam of sunlight 

 from a piece of looking glass so as to travel up the street at a most tre- 

 mendous velocity; but sui)pose that, instead of being moved by a mere 

 turn of the wrist, the mirror is made to rotate on an axle by mechanical 

 means at an enormous speed; then, Just as the rotation is more rapid, 

 so will the beam of liglit travel at a higher speed. In the particular 

 case that I am now going to bring before your notice, a small mirror of 

 hardened steel was made by Mr. Colebrook, the mechanical assistant 

 in tlie i»hysical laboratory at South Kensington, mounted so beautifully 

 that it would run at the enormous speed of 1,000 turns a second (not 

 1,000 a minute) without giving any trouble. The light from the spark 

 was focused by the mirror upon the photographic plate. Now, if the 

 light were really instantaneous, the image would be as clear and sharp 

 as if the mirror were at rest; if, on the other hand, it lasted long enough 

 for the image to be carried an appreciable distance, then the photo- 

 graph would show a band of light drawn out to this distance. The 

 mirror is now placed on the front of the platform, and a beam of electric 

 light is focused by it upon the screen, from which it is distant about 

 20 feet. Now that 1 turn the mirror slowly you see the spot of light 

 drawn out into a band reacliing across the screen, and this is described 

 over and over again as the mirror revolves. Let us suppose that the 

 mirror is revolving once a second, then it is easy to show that the spot 

 of light is travelling at about 250 feet a second. It is not difticult, there- 

 fore, to see that if the mirror is revolving 1,000 times as fast the spot 

 of light wmII traverse the screen 1,000 times as fast also, i. e., about 

 250,000 feet a second, or 100,000 miles an hour, a speed which is 200 

 times as great as that of a Martini Henry bullet, while such a bullet 

 only travels 14 times as fast as an express train. You will see, then, 

 that it is not difficult to observe how long a spark lasts when its image 

 can be whirled along at such a speed as this. I have now started the 

 electromotor, and the mirror is turning more and more rapidly. Now 

 it gives a musical note of the same pitch as that given by the tuning 

 fork I am bowing; it is therefore turning 512 times a second. It is now 

 giving a higher note, i. e., it is turning faster and faster, until at last it 

 gives the octave, at which time it is turning 1,028 turns a second. The 

 band of light on the screen is produced by a spot now travelling at a 

 still higher speed than that which I have just mentioned. 



I had hoped to have shown with this apparatus the actual experi- 

 ment of drawing out the apparently instantaneous flash of an electric 



