268 Dr. J. Robinson on 



largely of resonance radiation, the red and yellow region 

 being- analogous to the lines of the resonance spectrum. 

 The resonance radiation is weakened by the helium, the 

 energy going over to the red and yellow region, conse- 

 quently in helium at higher pressures the colour of the 

 emitted light is red. This total intensity of the fluorescent 

 light is reduced as well by helium at pressures above 10 mm., 

 and it is the combination of the two effects that gives us the 

 observed results. This wholly new effect of molecular col- 

 lisions upon the nature of spectra makes an excellent problem 

 for the theoretical physicists to solve. 



Berlin : Phys. Inst, der Universitat, 



December 1910. 



XXX. Electric Dust Figures. 

 By James Robinson, M.JSc., Ph.D.* 



IN a recent communication to this magazine Mr. Richmond f 

 described some dust figures obtained by the passage of 

 electric sparks at the end of a Kundt tube. Richmond 

 though u that possibly there was some connexion between the 

 distance apart of two ripples and the frequency of the electric 

 oscillation of the spark, but he failed to find any such direct 

 connexion. Such a view seems plausible, as it has been 

 shown % that an electric spark gives rise to short waves in 

 the atmosphere of the same frequency as the electric oscil- 

 lation. The following experiment shows, however, that the 

 ripples obtained in this way are not formed at the nodes of 

 stationary vibrations of these short waves, and that their 

 explanation must be sought in another direction. 



If the ripples do form at the nodes of such stationary 

 vibrations in the tube, then the distance apart of two ripples 

 ought to be independent of the intensity of the spark, and of 

 the diameter of the particles of the powder, when the electric 

 frequency is kept constant. An arrangement was suggested 

 to me by Prof. H. Stroud by means of which the intensity 

 of the spark could be varied without altering the frequency. 



A wire was taken from each of the spark-balls C of a 

 Wimshurst machine to one plate of each of two condensers 

 A and B. The other plates were joined through an in- 

 ductance D and also through a spark-gap E. On working 

 the machine, sparks passed simultaneously at C and E, The 



* Communicated by the Author. 



+ Phil. Mag. November 1909. 



% W. Altfoerg; Ann. der Phys. xxiii, p. 207 (1007). 



