306 Prof. Richardson and Lieut. Bazzoni on the 



in the body of the article, but on account of the width of the 

 slits are meant to be merely preliminary. It is of obvious 

 importance to increase the sensitivity of the arrangement to 

 such a degree that the minimum voltages required to excite 

 these new short radiations may be accurately determined. 



The statements contained in our note in 'Nature' 

 already referred to, are of some interest in this connexion. 

 We at that time stated that we had secured the many -lined 

 spectrum of helium at 22*5 volts, and that somewhat higher 

 voltages were required to bring out certain of the higher 

 frequency lines. The apparatus was, however, contaminated 

 with mercury, so that we could not get the helium spectrum 

 without previously exciting the mercury spectrum, which 

 made it possible to suspect that the presence of mercury 

 may have had something to do with the low potential 

 required to produce the helium lines. Shortly after publish- 

 ing these results we tried the experiment in pure helium 

 free from mercury and in a mercury-free apparatus. There 

 was no difficulty in maintaining the many- lined helium 

 spectrum at 22 volts to 22*5 volts under these conditions. The 

 same phenomena were observed as before with regard to the 

 successive appearance of certain of the lines according to 

 their frequencies as well as to the series in which they 

 belonged. There is no reason to suppose that with conditions 

 properly intensified it would not be possible to get this 

 spectrum at potentials lower than 22 volts. We have here 

 an analogy to the phenomena observed in mercury, where 

 Hebb has recently * obtained the many-lined spectrum down 

 to or below 5 volts, while a secondary ionization potential 

 seems, from the work of Franck and Hertz and others, to be 

 established at 4*9 volts and a primary ionization potential at 

 about 1025 volts. By primary ionization potential we 

 mean the potential at which heavy ionization sets in. These 

 facts all strengthen the supposition that the impact ionization 

 potential is lowered by absorption of radiation. 



It should be added that tests were frequently made with 

 the different gases experimented with to see if there was any 

 radiation of appreciably shorter wave-length than the limits 

 given above, by applying larger magnetic fields. These 

 tests were carried up to fields corresponding to about 

 60 A.U., but a complete record of the tests has not been 

 made as they were uniformly negative. In the case of 

 helium we have records of such tests under various conditions 

 made at 3'0, 3*5, and 4*0 amps, or H = 60, 70, and 80 lines. 



* Physical Review, May 1917. 



