290 Mr. E. W. B. Gill on the Electrical 



first half-oscillation the metallic streamers are more intense 

 at the electrode which is for the instant negative. 



4. The velocity of the metallic vapour has been measured 

 from the slope of the first edge of the initial streamer. The 

 velocity falls off with increase of self-induction; but it was 

 possible to show that this is due to a reduced temperature. 



5. Two simultaneous velocities in one and the same spectral 

 line are seen in the case of calcium vapour. 



I wish to express my indebtedness to Prof. Schuster and to 

 Prof. Rutherford for their interest in these experiments and 

 for many suggestions. 



Physical Laboratory, 

 Manchester University. 



XXVIII. TJie Electrical Effect of the Ultra-violet Spectrum. 

 By E. W. B. Gill, B.A., B.Sc; Fellow of Merton College, 

 Oxford*. 



WHEN ultra-violet light falls upon a metal plate it is 

 well known that negative ions are set free at the 

 surface of the plate, the effect being most marked in the case 

 of zinc. The present investigation was undertaken to ascer- 

 tain the relative powers of each part of the ultra-violet 

 spectrum for setting free negative ions. 



The existence of ultra-violet light was first discovered by 

 its acting upon silver chloride, and most of our knowledge 

 of the ultra-violet spectrum has been obtained by photo- 

 graphing it. The discovery was soon made that glass was 

 opaque to these rays, and it was found necessary to use quartz, 

 or better still fluorite, for the prisms and lenses used in pro- 

 ducing the spectrum. The early photographic work consisted 

 simply in detecting various lines in the ultra-violet spectrum, 

 and it was not till gratings were emplo3 r ed that the wave- 

 lengths of these lines were determined. If the spectrum be 

 produced by a quartz prism, the absorption of rays by the 

 quartz and also by the air gets very considerable as the 

 waves get shorter. Schumann f investigated the absorptive 

 power of quartz and found that a plate 2 cm. thick cut off 

 all wave-lengths shorter than 185 /*//,. 



For the purposes of this investigation the most important 

 measurements were those of Rubens J, who found for the 



* Communicated by Prof. J. S. Townsend, F.R.S. 

 t Wien. Ber. cii. II. A. p. 415 (1893). 

 t Wied. Ann. liv. p. 476. 



