INTRODUCTION. 5 



of light causes the lines to be slightly shifted towards the red. Humphreys 

 considers this to be due to the magnetic fields of neighboring atoms. Rich- 

 ardson, 1 on the other hand, considers the shift as due to electrostatic action. 

 An increase of the partial pressure of the vapor of the emitting substance 

 only causes the lines to widen. An increase of the total pressure of the 

 surrounding vapor causes a shift, and this Richardson considers to be due 

 to sympathetic vibrations set up in the surrounding atoms. If an atom 

 is emitting light, it must be surrounded by an alternating field of force, 

 and this will produce forced vibrations of equal period and, under certain 

 conditions, of equal phase in the neighboring atoms. These sympathetic 

 vibrations will then react upon the emitting atom and increase its period. 

 After making several assumptions as to the vibrator in the emitting atom, 

 Richardson deduces a shift which is considerably larger than that observed. 



The second phenomenon of the change of frequency of line-spectra is 

 that of the Zeeman effect. Many lines show a simple Zeeman effect such 

 as would be produced upon a vibrating negative electron. Other lines show 

 a very complex Zeeman effect which as yet has not been fully explained. 

 Still other lines show no Zeeman effect at all. All line-spectra show a 

 Zeeman effect that indicates that the vibrator carries a negative charge. 

 Series lines usually show a similar resolution in a magnetic field as well as 

 a similar behavior under variations of pressure, temperature, etc. Very 

 important discussions of the Zeeman effect by Lorentz, Voigt, Ritz, etc., 

 have recently appeared. 



Some interesting work has recently been done by Lenard, 2 Stark, 3 and 

 some others on the carriers of matter that are emitting light. In vacuum- 

 tubes, in arcs, in flames, and in the radiations from radioactive bodies we 

 have electrons, atoms, molecules, charged atoms or molecules, or aggrega- 

 tions of these that are moving in some cases with very great velocities. The 

 free electron, as we shall see, radiates a continuous spectrum, but the bound 

 electron, being disturbed comparatively infrequently by collisions of the 

 atom in which it is bound, will emit more or less monochromatic radiations. 

 Now, in the case of flames, arcs, etc., it is possible to separate the positively 

 and negatively charged ions by means of an electric field. 



Lenard's work indicated that the radiators of line-spectra were either 

 neutral atoms of positive ions, the principal series being due to neutral 

 atoms, and the subordinate series to positively charged atoms. If an ion 

 has a swarm of molecules about it, it is unable to radiate or absorb. Stark 

 studied the Doppler effect of canal-rays in vacuum-tubes. He found that 

 in the case of a beam of light coming from the canal-rays in the same direc- 

 tion in which they are moving, many spectrum lines showed a "rest" line 

 and a displaced line due to the Doppler effect. He found that some series 

 lines consisting of doublets originate from univalent positive ions and the 

 mercury triplets start from divalent positive ions. Some lines show no 

 Doppler effect and these originate from a negative electron joining a positive 

 ion. The displaced line is separated from the normal line by a dark space 



1 Phil. Mag., 14, 557 (1907). 



2 Ann. Phys., 9, 642 (1902); 11, 649 (1903); 12, 475, 737 (1903). 



3 Ibid., 14,506 (1904). 



