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



an angle of nearly one hundred and eighty degrees during 

 the existence of a spark, while to turn it through a right 

 angle is perfectly easy, each lens will project a beaded arc of 

 a circle perhaps nearly half a turn in extent. If the separate 

 lenses were all at the same distance from the axis, these six 

 semicircular arcs would be projected on the top of one 

 another and confusion would result; but as they are displaced 

 by an amount which is about the same as the length of a 

 convenient spark, the separate arcs are just made to clear one 

 another. Therefore, if the plate is large enough to take an 

 arc of the length of sixty degrees, the whole record of the 

 spark will afterwards be found upon it. By this means it is 

 possible to make use of the high speed which is necessary to 

 widely separate the several constituents of the oscillating 

 spark without at the same time causing the record to be 

 spoiled by the overlapping of the image formed by one lens 

 by that produced by the next. A single pair of lenses, in- 

 stead of six, would prevent the overlapping, but then to 

 certainly obtain the whole record it would be necessary to 

 make the plate receive an arc of one hundred and eighty 

 degrees, which, on optical grounds, would be objectionable. 

 If the sparks are to be shown on a screen it is best to make 

 the spark in the axis of rotation, and show the complete circle 

 or a large part of it ; but if the image is to be photographed, it 

 is test to place the spark exactly opposite one lens and use 

 such a size of plate that when the disk is turned one image 

 just begins to leave the plate on one side when the next is 

 coining in on the other. 



In order to make the most of a plate, it is convenient to 

 expose it in a common camera-slide held in the hand or in a 

 groove behind a card screen, with a curved aperture of such 

 a size that the image from all three lenses can fall upon the 

 plate. The plate can then be moved on by hand one stage after 

 every spark, and five or six groups taken on one half plate. 

 Specimens of portions of a few T of the photographs obtained are 

 arranged together in PL VI. fig. 6. 



Dr. Lodge has lately photographed oscillatory sparks, 

 and has observed their peculiarities; 1 shall therefore do no 

 more than merely mention the most striking features. The 

 spark is obviously alight nearly all the time that it lasts, the 

 intervals of darkness are short. The oscillatory spark proper 

 is, I believe, always determined by what may be called a pilot- 

 spark, due to the discharge of the knobs and wires outside the 

 self-induction. 



The pilot-spark is well seen at p in A, fig. 6, which is a 

 photograph taken with a very high speed and considerable 



