﻿Spectra of Helium and Hydrogen. 291 



passing through the primary of the induction-coil was 

 adjusted until the cathode was almost white-hot, and it was 

 kept at this temperature for nearly four hours. By reversing 

 the direction of the current passing through the tube, the 

 other electrode was treated in a similar manner. This 

 process was repeated until no hydrogen lines were visible 

 in the Hilger direct-reading spectroscope. The condenser 

 discharge was then passed through the tube for 30 minutes, 

 and both the bulbs and capillary were examined for the 

 presence of the hydrogen lines. If the lines eventually 

 appeared, the process of heating the electrodes was continued 

 until the spectrum showed no hydrogen lines after the 

 passage of a heavy condenser discharge for one hour. During 

 these experiments the platinum electrodes spluttered to the 

 sides of the spectrum-tube and consequently became much 

 finer. This was also made evident by the bending of the 

 electrodes to touch the sides of the bulbs when a heavy 

 condenser discharge was passed through the tube. In later 

 experiments, the diameters of the bulbs were so adjusted 

 that the platinum electrodes would not touch the sides even 

 when perpendicular to their original directions. In the pre- 

 liminary experiments, all the photographs and observations 

 were obtained with the spectrum-tube in the vertical position, 

 but Liter, especially when investigating the series repre- 

 sented by equation (9), the tube was usually placed in the 

 end-on position with the capillary pointing towards the 

 spectroscope. 



The spectrum obtained by passing the ordinary discharge 

 without condenser and spark-gap through a tube containing 

 helium at a pressure of '25 mm. was also studied. Under 

 these conditions a stream of charged particles, which were 

 deflected by a magnet in the same direction as cathode rays, 

 passed down the tube. The results of experiments carried 

 out in this way will be described later. 



For the determination of wave-lengths, the lines of the 

 ordinary helium spectrum were employed as standards ex- 

 cept in the red and yellow, where they are too far apart. In 

 these regions, the impurity lines of mercury, sodium, silicon, 

 and oxygen were found useful. Also in some cases the 

 wave-lengths were determined with the aid of a copper or 

 barium comparison spectrum. The mercury lines only 

 appeared on one of the plates, but the sodium, silicon, and 

 oxygen lines were always present on photographs taken when 

 a heavy condenser discharge was passed through the tube. 

 These lines are due to the decomposition of the" glass o£ the 

 capillary by the discharge. 



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