Fig. 3. 



252 Prof. C. V. Boys on Photographs of Rapidly Moving 



enlargement of one of these photo- 

 graphs. The water was broken up re- 

 gularly by simply blowing into a key 

 which whistled a note of 2730 complete 

 vibrations a second, so that each drop 

 moved to the position occupied by the 

 next in the ^yVo °f a second. Lord 

 Rayleigh photographed jets of water 

 by the spark with a camera and lens, 

 using in addition a large condensing 

 and field lens, so as to obtain sufficient 

 light ; but it appears that the glass of 

 the lenses is not of much advantage, if 

 any, because, as he himself has since 

 suggested, it absorbs so large a pro- 

 portion of the ultra-violet rays that 

 the gain by concentration is about 

 balanced by the absorption of the 

 most actinic rays. 



3. Having found by the shadow 

 method that there was an abundance 

 of light from a comparatively feeble 

 spark, I thought it possible that each 

 half period of an oscillating spark 

 might produce sufficient light to leave 

 a clear record on a rapidly moving 

 photographic plate of the shadow of 

 small and rapidly moving objects. To 

 put this to the test of experiment, the 

 following arrangement was made. In 

 the first place an oscillating spark was formed between a pair of 

 polished brass knobs about one tenth of an inch apart, by dis- 

 charging a condenser of large capacity through one or two coils 

 of gutta-percha-covered copper wire to give self-induction. 

 Details of the electrical arrangements will be given at the 

 end of these notes ; but I may state here that the separate 

 flashes of light in the spark followed one another at the rate 

 of about 4500 a second. A " half plate " was held in a recess 

 in a disk of wood screwed to a whirling-table, so that it could 

 be made to rotate in its own plane at a high speed. A card- 

 screen with a narrow slit was placed just in front of the plate, 

 so that the light from the spark could fall on the plate through 

 the slit. The jet of water was then formed just in front of 

 the slit, and when the plate was rapidly revolving the spark 

 was formed. The plate, on being developed, was found to have 

 impressed upon it a picture like an open fan with the water- 



