XI.] EVIDENCE AFFORDED BY THE SHIFTING OF LINES. 107 



Artificial Shifting of Lines. 



The " shifts " we have so far referred to are real, depending upon the 

 environment of the molecules the vibrations of which build up the spectra. 



But there are also what we may term artificial shifts, the observa- 

 tion of which has recently led Dr. Schuster and Mr. Hemsalech to 

 conclusions of great importance almost equalling those noted by 

 Messrs. Jewell, Humphreys and Mohler from our special point of view. 



To see the point of this new work, let us consider a strong jar spark 

 taken between two different metallic poles in air. What happens is 

 thus described. 



u The initial discharge of the jar takes place through the air ; it 

 must do so because there is at first no metallic vapour present. The 

 intense heat generated by the electric current volatilises the metal 

 which then begins to diffuse away from the poles ; the subsequent oscilla- 

 tions of the discharge take place through the metallic vapours and 

 not through the air."* 



Next let us assume that the vapours produced at each pole take time 

 to pass to the other. If we observe by means of a revolving mirror, 

 the spark qud air will give us a straight line, the spark gud each vapour 

 will give us curved lines. 



Next suppose that instead of observing the sparks thus produced 

 by the three different sources, we observe their spectra. This has been 

 done by Dr. Arthur Schuster and Mr. Hemsalech, who thus refer to it : 



" The method of the rotating mirror tried during the course of 

 several years in various forms by one of us, did not prove successful. 

 On the other hand good results were obtained at once on trying the 

 method used by Professor Dixon in his researches on explosive waves. 

 This method consists in fixing a photographic film round the rim of a 

 rotating wheel. All that is necessary for its success is to have sparks 

 so powerful that each single one gives a good impression of its spec- 

 trum on the film. Were the sparks absolutely instantaneous, the 

 images taken on the rotating wheel would be identical with those 

 developed on a stationary plate, but on trial this is found not to be 

 the case. The metal lines are found to be inclined and curved when 

 the wheel rotates, and their inclination serves to measure the rate of 

 diffusion of the metallic particles. The air lines, on the other hand, 

 remain straight, though slightly widened. 



" To avoid the tendency of the film to fly off the wheel when fixed 

 round its rim, as in the original form of the apparatus, a spinning disc 

 was constructed for us by the Cambridge Scientific Instrument Com- 

 pany. The film is placed flat against the disc, and is kept in place by 



* Free. Roy. Soc., vol. 64, p. 331. 



