Spectra of Helium, Sfc. in the Ultra- Violet. 287 



from hydrogen. It is especially bard to anticipate what ma}- 

 bo the effect of traces of hydrogen on the helium spectrum. 

 It is further to be expected that all radiation at all absorbable 

 in helium will be eliminated before arriving' at the photo- 

 graphic plate. On the other hand, the radiation detected 

 occurs in definite lines, the wave-length of which can be 

 determined with considerable accuracy. 



In the method which we have used the radiation is allowed 

 to fall on a metallic target, and the velocity of the photo- 

 electric electrons emitted is measured. It is thus possible to 

 determine the frequency of the impinging radiation, and 

 from the distribution of velocities amongst the emitted 

 electrons something of the distribution of frequencies in 

 cases where the radiation received is complex. In making 

 these determinations it is necessary to assume the correctness 

 of the equation ^mv 2 = e\ 7 = h(v — v ) and its validity in the 

 new region of very short waves. This equation has, however, 

 been established beyond any question on both sides of this 

 region. In the ultra-violet the researches of Richardson and 

 Compton * and of Hughes | have shown that it applies to a 

 very large number of very varied materials, and Millikan t 

 and his pupils have carried out tests with a few carefully 

 selected substances which show that it holds with very high 

 accuracy. The researches of Whiddington and others have 

 shown, though not so directly, that it also holds in the X-ray 

 region, and there is no reason for doubting its validity in the 

 intermediate part of the spectrum. Determinations of this 

 kind offer advantages over spectroscopic methods for this 

 particular problem in that they can be carried on in an 

 apparatus of small dimensions without windows or lenses, 

 and of such a character that it can be made and kept free from 

 occluded gases. 



An obvious method for measuring the maximum velocity 

 of the electrons lies in the use of a back potential to check 

 the emission. We tried this method with an apparatus we 

 have described in another connexion §, but the phenomena 

 were complicated by a large photoelectric emission which 

 took place from the walls of the collecting sphere surrounding 

 the target. Owing to this the results obtained by this method 

 were indecisive and consequently were not published. We 

 then determined to sort our. the electrons by bending their 

 paths into circles by a magnetic field and calculating the 

 velocity of emission from the value of the field necessary to 



* ' Science,' Mav 17, 1912 ; Phil. Mag. vol. xxiv. p. 575 (1912). 

 t Phil. Trans. A. vol. 212. p. 205 (1912). 

 t Phys. Ptev. vol. vii. p. 355 (1916). 

 § Phil. Mag. vol. xxxii. Oct. 1916. 

 X2 



