528 Professor Thomas R. Merton [March 10, 



trum becomes very much weaker. In a vacuum tube containing: 

 water vapour the lines of the Balmer series are extremely intense, 

 whilst those of the secondary spectrum are relatively very faint. 

 The investigations of Michelson and Lord Rayleigh, and of Buisson 

 and Fabry, have shown that under certain conditions the masses of 

 the atoms or molecules from which the spectrum originates may be 

 deduced from a knowledge of the widths of the spectrum lines, and 

 recent investigations, in which the widths of the lines of the secon- 

 dary spectrum of hydrogen have been measured to a high degree of 

 precision, have shown that the secondary spectrum is to be referred 

 to the hydrogen molecule. The presence of impurities in vacuum 

 tubes containing hydrogen not only enhances the lines of the Balmer 

 series, but also brings about changes in the relative intensities of the 

 Balmer lines themselves. Some of these changes are very striking, 

 but there are other variations of a more subtle kind, which are only 

 discovered when accurate quantitative measurements are made of the 

 relative intensities of the lines. A most striking effect is observed 

 when a relatively large quantity of helium is admitted to a vacuum 

 tube containing hydrogen. Under these conditions the relative 

 intensities of some of the lines of the secondary spectrum alter in a 

 surprising manner, some of the lines being greatly enhanced, whilst 

 others become very weak. 



From a theoretical point of view the spectrum of helium is second 

 in importance to that of hydrogen only. The lines of helium are 

 prominent in the spectrum of the chromosphere of the sun and of 

 many stars, and their relative intensity varies under different condi- 

 tions of excitation in the laboratory and in different celestial spectra. 

 There are six chief series of lines in the spectrum of helium, three 

 of which are usually referred to as the helium and three as the 

 parhelium series. The helium series are the stronger in vacuum 

 tubes containing the gas at pressures exceeding a few millimetres, 

 whilst at very low pressures the parhelium series are predominant ; 

 and since the chief visible line of the helium series is yellow, and 

 that of the parhelium series green, the colour of the discharge is 

 changed from yellow to green when the pressure is reduced. There 

 is another spectrum associated with helium which is analogous to the 

 secondary spectrum of hydrogen in that it only appears with any 

 considerable intensity when the gas is exceedingly pure. This 

 spectrum is known as the band spectrum of helium, and its occur- 

 rence in a gas which is known to be incapable of forming molecules, 

 in the chemical sense of the word, is very remarkable, in view of the 

 fact that band spectra are generally attributed to molecules. It 

 may perhaps be suspected that there is some temporary association 

 of atoms during the passage of the electric discharge which cannot 

 be referred to as a molecule in the chemical sense of the word. 

 Professor Fowler has shown that the arrangement of the heads of 

 the bands in this spectrum resembles that found in series of lines- 



