1916] on The Spectra of Hydrogen and Helium 645 



it is now generally conceded that this sccondarj spectrum really 

 belongs to hydrogen. The spectrum is so complex as almost to 

 defy analysis, but an important beginning has been made by 

 Fulcher, who has shown that some of the constituent lines in the 

 green and red are arranged like those of a band spectrum. These 

 particular lines are relatively stronger at low pressures, and their 

 near equality of step is clearly seen in small-scale photographs. 

 On current theories, these band lines would be considered to 

 emanate from hydrogen molecules, but it is not yet certain that this 

 is true of all the lines of the secondary spectrum, as some of them 

 show considerable Zeeman effects in the magnetic field. The 

 secondary spectrum offers less attraction to astronomers than to 

 physicists, for it has not yet been recognized in any celestial body 

 whatsoever. 



The primary spectrum of hydrogen is of more immediate interest. 

 It may be obtained conveniently from an ordinary vacuum tube 

 having a narrow capillary, if the gas be at a pressure of a few milli- 

 metres. Sealed tubes, however, soon deteriorate, in the sense that 



<^ t 



Fig. 1. — Balmer Series of Hydrogen Lines. 



they give undue prominence to the secondary spectrum at the 

 expense of the primary. Mr. Guild, of the National Physical 

 Laboratory, has lately shown that this arises from increase of pres- 

 sure caused by the evolution of gas from the electrodes, and that 

 the life of a tube intended for the primary spectrum may be much 

 prolonged by the addition of a large bulb, so that the rate of increase 

 of pressure is much reduced. You will observe that the red glow of 

 this tube is in striking contrast with the blue of the tube which gave 

 us a strong secondary spectrum. 



When we examine the primary spectrum, we are at once struck by 

 the wonderful regularity in tlie arrangement of the lines (Fig. 1). 

 AVe see how the lines gradually become closer together and diminish 

 regularly in intensity as we pass from the red to the ultra-violet. It 

 was not until 1885, however, that the law connecting the lines was 

 discovered. It was then found by Balmer that the wave-lengths of 

 the known lines could be accurately expressed by the simple formula 



A = 3646-14 -^J^ 

 nv - 4 



