Objects, and on the Oscillating Electric Spark. 253 



drops arranged in spiral lines. This method was not so con- 

 venient as a modification in which, instead of a glass plate 

 spinning in its own plane, a flexible film was employed, 

 wrapped round and held by two india-rubber bands upon a 

 brass cylinder 1 J inches in diameter. This could be made to 

 rotate at a very high speed by means of a light band from a 

 whirling-table. PI. VI. fig. 4 is a copy of a photograph obtained 

 in this way. Here it will be seen that the intermittent light 

 is automatically produced, and the period of the intermittence 

 is constant for any particular apparatus. However, as carried 

 out, the value of this method of observing the movements 

 of rapidly moving bodies loses its practical value because the 

 spark is alight during a large proportion of the time of each 

 oscillation, so that it is by no means an instantaneous 

 phenomenon. 



4. In order to show the oscillating spark and to examine it 

 by photography, I wished to have some more convenient 

 means than the rotating mirror employed by Dr. Lodge. 

 With this the observer is taken by surprise whenever a spark 

 passes, for he does not know exactly in what direction to look, 

 and therefore he finds it more difficult to recognize the 

 peculiarities of the drawn-out spark than he would if he were 

 certain that he would see the spark in the exact direction in 

 which he was looking. Besides this it is not easy, when 

 photographing the spark, to ensure that the whole phenome- 

 non from the birth to the dying flickers of every spark shall 

 be impressed upon the plate. 



The method which I am about to describe appears to me to 

 attain these ends more perfectly than any other with which I 

 am acquainted, and it does not introduce any difficulties 

 of its own. 



A brass disk (PI. VI. fig. 5) has six achromatic opera-glass 

 lenses of about seven inches focus screwed into it, but not at 

 the same distance from the axis. One pair, 1, 1, on opposite 

 sides of a diameter are placed equidistant from the axis \ the 

 next pair 2, 2 are placed one tenth of an inch nearer the axis ; 

 the third pair 3, 3 one tenth of an inch nearer still. The 

 wheel is mounted on an axle, carrying a grooved pulley-wheel, 

 and is accurately balanced. It may then be made to rotate, if 

 necessary, at the very high speed of over a hundred turns a 

 second. The spark is formed on one side of the plate, and a 

 screen or photographic plate is held on the opposite side ; and 

 the two are so adjusted that the spark is focused upon the 

 screen or plate. Now, when the disk is made to rotate, the 

 image of the spark is drawn out into a beaded band ; but as it 

 is possible to turn the disk so fast that it has moved through 



Phil. Mag. S. 5. Vol. 30. No. 184. Sept. 1890. T 



