708 



Prof. R. W. Wood 



Neu 



air molecules or microscopical dust particles, as in case of 

 blue-sky ; (b) ultra-violet fluorescence of the air caused by 

 the absorption of the Schumann waves. As a matter of fact 

 neither hypothesis turned out to be tenable, but I mention 

 them to show that they have been carefully considered. 



If the emission of ultra-violet light by the air was merely 

 a scattering, its spectrum should be identical with that of the 

 spark : if, on the contrary, it is a fluorescence phenomenon, 

 its spectrum would be totally different. In the spectro- 

 scopic work it was necessary to get as close as possible to the 

 spark, and yet run no risk of having its direct light enter 

 the slit of the instrument. To meet this requirement the 

 apparatus shown in fig. 1 was constructed. A disk of 



Fig. 1. 



aluminium, 3 mm. thick, was perforated with a hole 1*5 mm. 

 in diameter and fastened to a short brass cylinder B. The. 

 aluminium electrode C was carried by a screw D, which 

 passed through the ebonite cap E. The spark discharge 

 passed between C and the inner rim of the hole in the 

 aluminium disk. If the spark chamber is hermetically 

 sealed the explosive expansions of the air are apt to force 

 the spark aureole, which is pale green in the case of the 

 aluminium spark, out through the hole. The small lateral 

 tube J prevents this, and serves as well for the introduction 

 of various gases. The length of the spark must be so 

 adjusted that no visible portion is forced out through the 

 hole, when viewed from the position K. 



A second brass cylinder F, closed at the top and fitted 



