THEORY AND FORMER INVESTIGATIONS. 5 



can be made as small as we please and brought within the errors of measurement. Runge gives a cri- 

 terion as to how far it is allowable to go in such calculations. This question of commensurability will 

 receive attention in the following study of the iron and titanium spectra. 



Dissymmetry in the separation and in the intensity of components on the red and violet sides has 

 been observed many times in Zeeman investigations. Voigt (3/') arrived at the conclusion that light 

 observed at right angles to the force-lines should give a triplet whose red component is slightly closer to 

 the central hne and stronger than the violet component. Observations by Zeeman (30) on the iron 

 spectrum gave a number of cases where such a dissymmetry seemed to exist. Reese (12) also found triplets 

 and lines of higher separation for several elements which appeared to show the effect. More recently 

 a series of papers has been published by Zeeman (30 comparing the mercury triplets XS770 and XS791 

 by various optical methods. The latter line is distinctly shown to have its red component nearer the 

 central line than is the violet component, while X5770 remains perfectly symmetrical. The amount of 

 dissymmetry appeared to vary as the square of the field-strength. This confirmed a measurement made 

 about the same time by Gmelin (32) with the echelon grating. A dissymmetry of this sort is always small 

 and difficult of detection. Large dissymmetries are to be classified as abnormal separations. A few lines 

 of such a character occur in the iron and titanium spectra, which will be noted later. Lines of very pro- 

 nounced dissymmetry were measured by Jack ( 33) in the spectra of tungsten and molybdenum. Chromium 

 also shows a great number of unsymmetrical separations. Some striking cases were observed by Dufour (34) , 

 and many others have been photographed in this laboratory. The theory of coupled electrons, by which 

 Voigt (35) has sought to explain complex separations in general, allows for the occurrence of such dissym- 

 metries. 



The magnetic separation of absorption hues, or the "inverse Zeeman effect," has been investigated 

 by a number of observers, as a rule for only a few hues. In such experiments white light is passed through 

 the vapor of a luminous source placed between the poles of a magnet. It was shown by Konig (36) and 

 Cotton (37) that there is a full correspondence between the effects of the magnetic field for both emission 

 and absorption lines. The splitting of lines in the spectra of sun-spots observed by Hale (38) was thus 

 proved to be due to the action of magnetism by comparing the Zeeman effect for the same lines as pro- 

 duced in the laboratory. The peculiarities in separations of sun-spot lines can thus be studied, as is being 

 done in this laboratory and by Zeeman and Winawer (39) in their investigation of special polarization 

 effects for absorption lines, especially when the light passes at different angles to the magnetic force-lines. 



2. Possible Relation Between Zeeman Separation and Pressure Displacement. 



A preliminary paper on this subject has been published by the author (40). In the discussion of the 

 present results material will be offered for an extended study to test the hypothesis of a direct connection 

 between the Zeeman effect and the pressure displacement for spectrum lines. That such a relation exists 

 has been strongly advocated by Humphreys (41) in a series of papers which have been summarized (42) by 

 him, together with all other pressure investigations up to the year 1908. Humphreys's hypothesis, briefly 

 stated, is that the part of the atom to which the light impulse is due is a ring of electrons, rotating with 

 a period of the order of the light vibration. Each of the electron rings will then set up a magnetic field 

 of its own. The luminous gas will be in a condition of minimum potential energy when the planes of 

 the rings are parallel and the electrons revolving in the same direction. We must, however, in view of 

 the Zeeman effect, consider that different rings may rotate in opposite directions, and assume merely 

 that the regular condition is a rotation of the electrons in orbits approximately circular, with a tendency 

 for the planes of these to become parallel. The effect of pressure in the surrounding medium will be to 

 bring the rings closer together, thereby altering their mutual induction. If two rings rotating in the same 

 direction are made to approach, the current in each ring will decrease, which means a retardation of the 

 rotating electrons and an increase of period in the corresponding light vibration, resulting in a shift of 

 the spectrum hnes toward the red. ^ 



