Fluorescence of Gases excited by Ultra- Schumann Waves. 899 



The spacing of the (111) planes 



2a 

 a/3 

 = 1-738 xmh 



Therefore A, the wave-length of the larger constituent of 

 the rhodium doublet 



= 2d . sin0. 



= 3-476 x 0-1494 x mh. 



= 0-5194 X mi. 



If we take m = l'64x 10~ 8 , we find that 



A = 0-614xl0" 8 . 



Summary. 



An account is given of experiments in which the intensities 

 of reflected X-ray pencils of various orders are compared 

 with each other, and the results are considered in reference 

 to the recent theoretical discussion by Debije. 



It appears that there is a striking agreement between 

 theory and experiment in the case of rocksalt. 



The difficulties of experiment with the diamond are also 

 discussed. The existence of an X-ray absorption band in 

 the diamond is demonstrated. The experiments give oppor- 

 tunity for an accurate determination of the wave-length of 

 the rhodium X rays. 



XCVIII. The Fluorescence of Gases excited by Ultra- Schumann 

 Waves. By R. W. Wood and Gr. A. Hemsalech *. 



[Plates XIV. & XV.] 



AN investigation was made a number of years ago by one 

 of the writers, with a view of detecting a possible 

 ultra-violet fluorescence of air excited by waves in the 

 Schumann region f. 



A small hole was drilled through a plate of aluminium 

 and condenser-sparks discharged against the under side at 

 the perforated spot. If the region above the plate was 

 photographed with a quartz lens in a dark room, it was found 

 that the air above the hole was emitting ultra-violet light, 



* Communicated by the Authors. 



•f- Wood : " A new Radiant Emission from the Spark," Phil. Mag. 

 [6] vol. xx. p. 707 (October 1910). 



