286 Prof. Richardson and Lieut. Bazzoni on the 



and 20 volts* respectively to be correct, the limiting wave- 

 lengths come out as 1188 X 10 -8 cm. and 618 x 10" 8 cm. 

 When these experiments were begun in 1915 no " ultra- 

 violet " radiation shorter than 900 x 10~ 8 cm. had been 

 detected. On the other hand, various measurements with 

 X-rays had shown the maximum known wave-length of 

 these to be in the neighbourhood of 1 x 10~ 8 cm. It seemed 

 from the considerations given above that the investiga- 

 tion of the radiation from helium promised to fill a large 

 part of this gap in the radiation spectrum. To carry on 

 work in the neighbourhood of A, 400 it was, however, 

 necessary to devise a new method of experimentation. It 

 is to be observed that difficulties arise not only because such 

 short waves appear to be absorbed completely in all solid 

 media, but also because the radiation corresponding to the 

 ionization potential may be expected to be highly absorbable 

 by the gas giving rise to it. To detect such radiation it is 

 therefore necessary not only to eliminate all lenses, prisms, 

 and other absorbing bodies but it is also desirable to avoid 

 the long path through the gas demanded by vacuum grating 

 spectrographs. The gas must be worked with at a low pres- 

 sure, and preferably with low and definitely known applied 

 potentials. It is further necessary in dealing with helium to 

 have an experimental arrangement in which the gas can be 

 maintained of extremely high and dependable purity. 



The previous investigations in the region of very short 

 waves have been made by Schumann**", who extended the spec- 

 trum from A 1850 to A, 1230 bj the use of a vacuum fluorite 

 camera-spectrograph, and by Lyman "f, who in 1914 by the 

 use of a vacuum grating camera-spectrograph extended the 

 spectrum to A 900. Since the experiments dealt with in this 

 paper were begun Lyman has published § the records of work 

 carried on in helium in the apparatus above referred to 

 which give definite lines down to about A 600. This spectro- 

 graph is constructed of brass with greased joints. The 

 radiation path is something over 2 metres in gas at pressures 

 ranging from 1 to 3 mm. A disruptive discharge is used. 

 Under these conditions it is extremely difficult, if not im- 

 possible, to keep the helium free from impurities, particularly 



* The data and curves given "by Bazzoni in Phil. Mag 1 , vol. xxxii. 

 p. 566 (1916) make it improbable that the ionization potential of helium 

 in this neighbourhood exceeds 20 volts. However, Franck and Hertz 

 found a value 205 volts. If this is used the corresponding wave- 

 length is 603 X 10~ 8 cm. 



t ' The Spectroscopy of the Extreme Ultra- Violet,' by T. Lyman 

 p. 75 (1914), 



% Op. cit. p. 78. 



§ Astrophysical Journal, xliii. No. 2, March 1916. 



