THE SPECTRUM OF HYDROGEN 243 



not result in the emission of the Balmer hnes, but it is stated 

 that they are present in the flame issuing from a Bessemer 

 converter. 



As has been mentioned, there are other series closely associ- 

 ated with the Balmer series. There is one in the infra-red, and 

 another in the far ultra-violet, which can only be photographed 

 in a vacuum spectrograph, since a thin layer of air absorbs 

 completely light of such wave-lengths. Yet another series 

 has recently been announced in Nature from Prof. R. W. Wood's 

 laboratory. It lies in the far infra-red, and its existence was 

 predicted, and the wave-lengths calculated, from the previously 

 known series, from a theoretical standpoint. 



The Balmer series has been of fundamental importance to 

 the development of spectroscopic theory. It was in this 

 spectrum that a mathematical relation was first found which 

 would connect the wave-lengths of the constituent lines. From 

 the early days of the spectroscope spasmodic attempts were 

 made to discover some law to govern the spacing of the lines 

 observed, and a few fragmentary regularities were soon detected. 

 It was not until 1885 that Balmer discovered a formula which 

 expressed — and with surprising accuracy — all the members of 

 the series spectrum of hydrogen. His formula, discovered quite 

 empirically, is given by 



X = ^ X —„ X 10 ' 



ni — 4 



where X is the wave-length of the line. It is a constant, and m 



is given successively the integral values 3,4, 5,6, , to 



calculate the individual lines. The strongest line is in the 

 red (Ha, or the C line) and is obtained by putting m equal to 

 three. The lines represented by higher integers obviously get 

 closer and closer together towards the ultra-violet, and their 

 intensity diminishes very rapidly in the same direction. Only 

 the first eleven members were known to Balmer, but now more 

 than thirty have been traced in stellar spectra. This number 

 has not been approached in the laboratory, but Prof. Wood 

 by using a vacuum tube of a peculiar type has succeeded in 

 photographing twenty members. The Bohr theory suggests 

 that an upper limit may be set to the number of lines of this 

 series which can be developed in a vacuum tube, as the 

 diameter of the electronic orbit required for the emission of the 

 higher members becomes comparable with the free path of the 

 molecules. 



It is outside the scope of this article to describe the advances 

 springing from Balmer's discovery, such as the disentanglement 

 of similar series from other spectra, and the recasting of his 

 formula into a generalised form by Rydberg and others. This 



