﻿4 MORRIS- A IREY, Determination of Wave-lengths. 



A Rowland concave grating of I metre radius was 

 mounted with a slit and camera in a substantial box of 

 "Rotguss." The adjustments could be carried out through 

 small manholes in the cover of the box. The camera was 

 so situated that half the plate was covered with the lines 

 extending from 4,500 to 2,800 A. The focus could then 

 be adjusted with the ordinary iron arc light, and any 

 shorter wave-lengths would be in focus on the plate down 



o o 



to 1,100 A and their value determined to within *iAby 

 applying the scale found from the larger waves. 



Considerable difficulty was experienced in obtaining a 

 good vacuum in spite of the close texture of the metal of 

 which the box was cast, and it was only after enclosing 

 the whole apparatus in a cement made by mixing asbestos 

 powder with water glass that the box could be exhausted 

 to a pressure of j— mm. of mercury. 



The photographic plates were prepared according to 

 an improved recipe which Schumann was kind enough to 

 communicate privately.* 



The source of light was an electric spark produced 

 between aluminium electrodes by the discharge of a 

 Leyden jar connected to an induction coil capable of 

 giving a 1 5 inch spark in air. 



The spark gap was situated about 5 cm. from the slit, 

 which it illuminated through a small window of fluorspar 

 at the end of the box. Afterwards, to eliminate the 

 effect of the 5 cms. of air, the electrodes were enclosed 

 in a tube of hydrogen, which is very transparent, 

 as Schumann found. Vacuum tubes were also tried 

 placed directly on to the box without any fluorspar 

 intervening. The whole apparatus was filled with 

 hydrogen, and exhausted to a pressure of 1 mm. of 

 mercury. The double line 1,860, 1,852 A appeared 



♦Since published in WieeU Aiinalai, 1902. 



