Spectrum of Helium in the Extreme Ultra- Violet. 815 



and second, by the use of a continuous current in the 

 discharge-tube the chance of impurities was minimized. 



The result of these improvements was the discovery of a 

 fairly strong line at 585 A.U. The existence of this radiation 

 was confirmed by an observation with the vacuum grating 

 spectroscope of 97 cm. radius, long used in this laboratory. 



It is interesting to note that the resonance potential 

 corresponding to 585, 21'2 volts, agrees rather with the 

 results of Franck and Knipping than with those of other 

 investigators. The significance of this fact will be con- 

 sidered presently. 



It may be concluded that, apart from the two lines already 

 mentioned as probably belonging to the enhanced spectrum, 

 but whose origin is even now somewhat doubtful, only one 

 line in the extreme ultra-violet can be ascribed to helium 

 with any certainty at present. 



The description of the experimental arrangements and a 

 detailed discussion of results follows : — 



The vacuum spectroscope containing the grating of 20 cm. 

 radius was so arranged that its joints could be closed with 

 Khotinski cement. This method of sealing together with the 

 small volume facilitated the production of an excellent 

 vacuum, and assisted in preserving the purity of the gas 

 under examination. The discharge-tube was of quartz, the 

 cathode being of aluminium and cylindrical in form, about 

 2*4 cm. long with a hole 5 mm. in diameter ; the anode 

 was of tungsten. The cathode was always placed at the end 

 of the tube near the slit. The helium was purified by char- 

 coal and liquid air, the spectroscope being protected from 

 mercury vapour by U tubes refrigerated in the usual manner. 

 The gas when originally prepared was free from impurities. 

 However, as it was impossible to heat the whole apparatus, 

 the helium not infrequently showed the presence of traces 

 of hydrogen and the oxides of carbon when examined after 

 it had been admitted to the spectroscope. 



The gas was usually at a pressure of eight tenths of a 

 millimetre, the current being between twenty and forty 

 milliamperes. 



The dispersion of the grating was 84*6 A.U. to the mm., 

 the region contained in the length of the photographic 

 plate extending from the slit image to the neigbourhood of 

 1200 A.U. 



Under these conditions the strongest line on all our plates 



.U.; its intensity bears a constant ratio to 



that of the direct image of the slit. The other lines which 



are usually present at 686, 860, 972, 992, 1026, and 1176 



3 41 2 



