298 Professor Zeeman [March 80, 



to a line. Moreover, in this manner, we make direct use of a property 

 of the atom. 



You see here a vacuum tube with some mercury. We heat the 

 tube and excite it with the coil. You notice the briUiant Hght which is, 

 however, greatly increased when the tube is placed in a magnetic field.* 



For a given density of the vapour, there is a definite intensity of 

 field for which the luminosity is a maximum. You can see this when 

 we put on the current in the electro-magnet, the intensity of the 

 field then rises gradually. 



We project an image of the tube on the slit of a spectroscope. 

 This spectroscope must be so arranged that to every point of the slit 

 there corresponds a point of the image. The blue line of mercury 

 (4359) resolves into a sextet. Using this line the field of a du Bois 

 electro-magnet with a pole distance of 4 mm. is mapped out in the 

 spindle-shaped optical magneto-grams now shown. (Fig. 19.) We 

 may, of coarse, extinguish the light of the inner components. In 

 some cases a triplet will give more accurate results. The method 

 sketched will, of course, only be applied in difficult cases. As long 

 as our spectroscopes of great resolving power are rather cumber- 

 some, there is no practical application for the method. 



By means of this method we may also study some questions as to 

 the way in which certain phenomena, which accompany the resolution, 

 depend on intensity of field. 



We have no time, however, to discuss this further, because I 

 should like to refer to the important subject of the 



Behaviour of the Different Lines in the Magnetic Field. 



In many metallic spectra a number of lines occur which are 

 closely related and form so-called series of lines. The important 

 discoveries of Hartley, Liveing and Dewar were followed by the 

 discovery of series, owing to the indefatigable efforts of Balmer, 

 Kayser and Runge, Rydberg and Schuster. 



The plate shows diagrammatically the arrangement of the three 

 connected series which are found in the spectra of the alkalies and 

 other elements, and which are distinguished by Professor Schuster t 

 as the trunk series (Kayser and Runge's " Hauptserie "), the main 

 branch series (Kayser and Runge's " Zweite Nebenserie "), and the 

 side branch series (Kayser and Runge's " Erste Nebenserie "). 



The laws of these series are simpler than those governing acous- 

 tical vibrations. They are of an entirely different character ; for 

 instance, the members of each series approach some definite limit of 

 frequency, whereas the number of acoustical vibrations may increase 

 indefinitely. 



• Paschen. Physik. Zietschr., I. S., 478, 1900. 

 t Schuster. The Theory of Optics, p. 282, 1904. 



