A New Radiant Emission from the Spark. 707 



of X is accordingly 



T "V f ; 



it thus varies inversely as the square root of the length of 

 the interval of time. This result is of the same form as the 

 classical one used by E. v. Schweidler in the paper referred 

 to earlier. 



The probable value of | N— XT | (der durchschnittlicher 

 Fehler) is much more difficult to calculate. 



\ 



LXXVII. A New tiadiant Emission from the Spark. By 

 R. W. Wood, Professor of Experimental Physics in the 

 Johns Hopkins University *. 



[Plate XIV.] 



1 SCARCELY know how to designate the peculiar type 

 of radiation referred to in the present paper, which 

 was first discovered over two years ago in the course of some 

 experiments made with a view of ascertaining whether the 

 Schumann waves from the spark gave rise to any fluores- 

 cence of the air by which they were absorbed. It is now 

 known that there is a feeble ultra-violet luminosity of air or 

 nitrogen gas surrounding a small mass of radium, in other 

 words the radium renders the gas luminescent. To test for 

 a fluorescence due to the absorption of very short light- 

 waves, the condenser spark between aluminium electrodes 

 was passed behind and very close to a vertical strip of metal 

 which completely concealed the spark, but which enabled 

 observation, either visual or photographic, of the air in its 

 immediate vicinity. If the air in the room was free from 

 dust and smoke absolutely nothing could be seen with the 

 eye, even after prolonged resting in the dark. A photo- 

 graph, however, made with a small camera provided with a 

 quartz lens, showed that the air around the spark was a 

 source of a powerful actinic radiation, which was completely 

 stopped by the intervention of a glass plate between the 

 camera and the spark. The first photograph of the pheno- 

 menon which was obtained is reproduced on PI. XIV. fig. 6. 

 The narrow strip of metal between the two wider strips was 

 about 1 cm. in width ; the spark discharge was concealed 

 behind this. 



Two hypotheses immediately presented themselves : (a) we 

 are dealing with a scattering of the shortest waves by the 

 * Communicated bv the Author. 



3 A2 



