The Spectrum of Hydrogen Positive Rays. 241 



2. Method of Experiment. 



In order to obtain direct evidence on these questions, it 

 occurred to the author that it might be possible to combine 

 the electro-magnetic analysis of" the positive rays with an 

 investigation of their spectrum. The method first tried was 

 to analyse a narrow pencil of positive rays by magnetic and 

 electric fields in the ordinary way (see ' Rays of Positive 

 Electricity,' Sir J. J. Thomson, p. 20), and then examine 

 the spectra of the various constituents. It was found, 

 however, that the light obtainable from a pencil sufficiently 

 small to give sharp resolution was totally inadequate for 

 spectroscopic investigation, 



It was therefore decided to make use of the fact that by 

 changing the conditions in the discharge tube it is possible 

 to vary the proportion of atoms to molecules in the positive 

 rays over a wide range. 



The apparatus used is illustrated in fig. 1. A pencil 

 of positive rays from the discharge tube " A ; ' passed 



Fig. 1. 



i i * ' 



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GO 



j, To gaede 

 pump 



^Jfmmersed in 

 liquid air 



through an aluminium tube of 8 mm. diameter inserted in 

 the cathode " B," the end of the tube and the cathode being- 

 crossed by a network of aluminium wires. The positive 

 rays then passed into the observation chamber " C," through 

 the glass walls of which they could be examined in a Hilger 

 direct reading spectroscope. They then impinged on the 

 end of a fine copper tube (bore '35 mm.) ; those of the 

 rays which passed down this were deflected by electrostatic 

 and magnetic fields produced between the plates " D," and 

 thus all corresponding to the same value of e/m became 

 generators of one parabolic cone. After passing through 

 the chamber "E," in which the pressure was reduced to the 

 lowest possible amount by the use of charcoal cooled by 

 liquid air, they struck the willemite-covered glass plate " F," 



