of Emission and Absorption Lines in a Gas-Spectrum, 269 



The changes of position of the bright lines in the spectra 

 of metals due to changes in " pressure " were first investi- 

 gated by Humphreys and Mohler *. Their experiments 

 were suggested by some remarks of Jewel, who noticed that 

 the position of the metallic lines in the solar spectrum did 

 not correspond exactly to the positions of the same lines in 

 spectra obtained from an arc in the laboratory. In 1907 

 Humphreys published the complete results of a very exten- 

 sive research carried out by himself on this subject f. His 

 method consisted, generally speaking, in comparing the 

 metallic spectra obtained, from arcs surrounded by ordinary 

 air at pressures varying between 1 and 100 atmospheres. 



Theoretically very little has been done on the subject. 

 Three distinct explanations have, however, been suggested. 

 The earliest is that the phenomenon is due to the effect of 

 damping of the vibrations to which the emission of light is 

 due : this has been discussed at great length by Lommel and 

 WilsingJ. The second and apparently much better sug- 

 gestion explains the shift as due to the surrounding gas. 

 Prof. Larmor, in a highly suggestive and interesting note in 

 the ' Astrophysical Journal' (1907), attributes the shift to a 

 change in the specific inductive capacity of the surrounding 

 air. The third explanation bases the phenomenon on some 

 curious magnetic properties of the atoms. This was done in 

 order to explain some supposed connexion between the 

 phenomenon here under discussion and the Zeeman effect. 



I now propose to develop a theory which attributes the 

 effect to a density change in the actual incandescent vapour. 

 In another part of this paper I shall give my reasons for 

 believing that in Humphreys' experiments an increase in the 

 density of the incandescent metallic, vapour takes place 

 simultaneously with an increase in the pressure of the 

 surrounding atmospheric air. 



The theory here given is a simple explanation based on 

 the electron theory of optical effects first started and syste- 

 matically developed by Lorentz and Drude. The peculiarity 

 of the work here presented consists mathematically in the 

 introduction into the equations of motion of the single 

 typical electron of a force on that electron due to the polari- 

 zation in the surrounding medium. This term was, I believe, 

 originally introduced by Lorentz, and by means of it he 

 obtains the law (first discovered by Lorenz) connecting the 



* Astrophysical Journal, 1896. 

 f Astrophysical Journal, 1907. 

 X See Astrophysical Journal, 1893. 



