190 



MESSRS. A. SCHtfSTER AND G. HEMSALECH ON 



where each spark is of sufficient intensity to give a good spectrum. A sensitive film 

 is attached to a spinning wheel and the spark image formed on it. Were the spark 

 absolutely instantaneous, the images taken on the rotating wheel would be identical 

 with that taken on it when stationary, but on trial this is found not to be the case. 

 The metal lines are found to be inclined and curved, and their inclination serves to 

 measure the rate of diffusion of the metallic particles. 



The interpretation of the photographs presents no difficulties if the velocity of the 

 film is everywhere at right angles to the direction of the slit. Such is the case in 

 Professor DIXON'S experiment, and our first wheel was nearly a copy of that used by 

 him. Fig. 1 shows the section of this wheel ; the axle bears at A and B against 

 strong screws fixed in a cast-iron support, not shown in the figure. The rim of the 



Fig. 1. 



Fig. 2. 





wheel had a width of 4 centims., its diameter was 30 centims. The photographic 

 film was wound round the rim, and, although good results were obtained with this 

 wheel, some difficulty arose from the tendency of the film to fly off. Several 

 methods of fastening it to the rim were tried, but the only one which proved 

 satisfactory was to tie it down by two strong steel wires, stretched tightly round the 

 whole circumference. Velocities of about 80 revolutions a second could be obtained, 

 but at the high speeds the film lifted up along the central line, just where the spark 

 images fell. 



A new apparatus was now constructed for us by the Cambridge Scientific Instrument 

 Company, and is illustrated by figs. 3, 4, 5 (Plate 8), while fig. 2 shows a section of the 

 moving parts. The film is placed flat against a steel disc, CD, and is kept in place 

 by a second steel disc, EF, which presses against it, being secured by small screws, 



