On the Pressure Displacement of Spectral Lines. 499 



current; or conceivably the current has some small temporary- 

 effect on the gas itself, as is the case with oxygen when ozone 

 is formed. The quantitative evidence is contrary to both of 

 these methods of explanation, though less strong against the 

 second. Quantitatively, however, the readings are not very 

 reliable, and further investigation may show that one of these 

 explanations is correct. 



In conclusion I wish to express my gratitude to Professor 

 Townsend for the kindness he has shown in giving assistance 

 and advice in the course of these experiments. 



The Electrical Laboratory, Oxford. 



s 



LVI. On, the Pressure Displacement of Spectral Lines. 

 By R, Rossi, M.Sc* 



^INCB the discovery of the pressure displacement of 

 spectral lines several theories have been put forward 

 by different authors to explain this phenomenon. 



Schuster t first suggested that it ought to be ascertained 

 whether the displacement of the lines is due to pressure only, 

 i. e. to molecular impacts, or due to the proximity of molecules 

 vibrating with equal periods. 



Fitz Gerald % and Larmor §, treating the atom a<* an electro- 

 magnetic oscillator, found that an increase of specific inductive 

 capacity (due to an increase of pressure) would cause a dis- 

 placement of the spectral lines ; while Richardson ||, con- 

 sidering the forced vibrations set up in an atom by the 

 surrounding atoms, arrived at the same conclusion. 



Humphreys %, on the other hand, does not consider a change 

 in the specific inductive capacity of a gas to be the main cause 

 of pressure displacement. 



If the pressure displacement is a linear function of the 

 specific inductive capacity of the radiating vapour, the experi- 

 mental methods so far available are not accurate enough to 

 prove it on account of the small differences of the specific 

 inductive capacities of different gases ; but if if is proportional 

 to (yLt 2 — 1), as claimed by some authors, (fi being the refractive 

 index of the medium surrounding the vibrating atom), con- 

 siderable changes in the displacements ought to be detected 

 by surrounding the source of light with different gases. 



* Communicated by Prof. E. Rutherford. 

 t Astrophysical Journal, vol. iii. p. 292. 

 X Astrophysical Journal, vol. v. p. 210. 

 § Astrophysical Journal, vol. xxvi. p. 120 

 II Phil. Mag-. [6] vol. xiv. p. 557 (1007). 

 11 Astrophysical Journal, vol. xxvi. p. 18. 



