TllH SPECTRA OK STARS AND NKIIUM:. 



21 



specially strong spark discharges, as first shown by Lockyer 

 and liis assistants. 



A Pew of the B stars are further distinguished by a set of 

 linos of exceptional interest. They were first noted in the 

 spectrum of the star Zeta Puppis by Professor Pickering, who 

 attributed them to hydrogen, although they wore not found in 

 the laboratory spectrum of this gas. The ground tor this 

 interpretation was that the "Zeta Puppis Lines," or " Pickering 

 series," appeared to have a numerical relation to the ordinary 

 linos of hydrogen, such as had heen found to exist between 

 different systems of lines in the spectra of other elements. 

 This view was strengthened later by Kydberg's calculation of a 

 third, or " principal," series of associated lines, the first of which, 

 at wave-length 4686, was also found in stars in company with 

 the Pickering linos; the other members of this series occur in 

 the ultra-violet beyond the range transmitted by our atmos- 

 phere, and therefore could not be observed. 



On the basis of these observations and calculations, there 

 was for many years a widespread belief in the existence of a 

 form of hydrogen which was beyond the reach of laboratory 

 experiments. It was frequently called " cosmic hydrogen," or 

 " proto-hydrogen," and was thought to be a simplified form of 

 hydrogen produced only under the extraordinary conditions 

 which might be supposed to prevail in the hottest stars. About 

 two years ago, however, it was found by the lecturer that these 

 lines could be obtained by passing very strong discharges 

 through helium, the strongest discharges in -fact that glass or 

 quartz tubes will bear. 



In these experiments it is almost impossible to remove all 

 traces of hydrogen, and in the first instance the new lines were 

 attributed to hydrogen, in accordance with previous deductions. 

 Numerically, the lines were closely related to the hydrogen 

 series, ami had no apparent connection with the known lines of 

 helium. 



Later work, however, has shown, in accordance with certain 

 theoretical deductions made by Dr. Bohr, that the Zeta Puppis 

 lines are really due to a simplified form of helium, and do not 

 belong to hydrogen at all. They are now designated spark 

 lines of helium, or lines of " proto-helium." 



Those lines, first observed in the stars, are of great importance 

 in connection with theories of the structure of atoms, and 

 Dr. Bohr has given what appears to be a very satisfactory 

 explanation of the close agreement of some of them with the 

 lines calculated for "cosmic hydrogen" by Eydberg. The point 



