COMPARISON OF RESULTS FOR ZEEMAN EFFECT AND FOR PRESSURE DISPLACEMENT. 63 



For the medium and large values in each table, the proportion of lines increases in the region of greater 

 wave-length, this being very decided for the "large" group. Thus there is a clear increase in magnitude 

 of both separation and displacement as the wave-length increases. The lines here compared seem to be 

 representative of the spectrum, as the same relation holds in the complete Zeeman tables, which contain 

 a much larger number of lines for this range of wave-length. 



When pressure measurements of high accuracy are available for an extended region of wave-length, 

 the rate of variation with the wave-length will appear, and the closeness of agreement with the relation 

 found for iron and titanium, namely, that the magnetic separation increases proportionally with the 

 square of the wave-length (p. 54), will afford strong evidence concerning the common physical basis 

 of the two phenomena. An attempt at a comparison of this sort has been made by the author in a recent 

 paper (63) on the effect of pressure upon electric-furnace spectra. The displacements of iron lines given 

 by the electric furnace for a pressure of 9 atmospheres were measured for two regions 1000 A apart, from 

 X4050 to X 4450 and from X505o to X5450. The list for the latter region did not include as many of the 

 weaker lines, whose displacements are often large, as was available for the blue region, so that a compari- 

 son of the means of all displacements would not have been fair. It seemed best to hmit this preliminary 

 comparison to those lines in each region which show the same general behavior in various light sources. 

 In the furnace they appear at low temperatures and show reversal with strong widening under pressure. 

 They are lines which, although not connected by series relations, show such similarity in their response 

 to the excitations of furnace, arc, and spark that the vibrating particles which produce them can be 

 assumed to have many points of similarity. 



Fifteen lines of this character in the blue region were compared with nine similar hnes in the green. 

 The mean pressure displacement for the two sets was found to be almost identical, being 0.058 A for the 

 blue and 0.060 A for the green lines. The magnetic separations of the same lines, taken from Table i, 

 give mean values of 0.330 A and 0.520 A, respectively, for the blue and green regions, an increase of 60 

 per cent for a difference of wave-length of about 1000 A. The evidence from these selected lines is, there- 

 fore, against a close connection between the magnetic and pressure phenomena. Measurements for the 

 arc under pressure, however, show a more frequent occurrence of large displacements as we pass toward 

 greater wave-lengths, and more complete measurements will show the rate of change. 



Summarizing the comparison here presented, it may be said that there is a fair agreement between 

 magnitude of magnetic separation and pressure displacement for the lines of iron and titanium when the 

 means of large groups are considered. The number and character of the lines not in agreement, however, 

 show that the correspondence is not close enough to justify preferring any one of the theories for the 

 pressure effect on this ground, or to predict the eflect upon a given line of one influence from that observed 

 for the other. The degree of concordance which we have could perhaps result entirely from the fact 

 that the magnitude of each effect increases with the wave-length. This does not prove a close physical 

 relation, since any theory of the pressure effect that might be offered would probably involve a change 

 with the wave-length. A comparison of the rates of change of the two effects appears to be a more prom- 

 ising line of investigation than an extension of the method followed for iron and titanium; as the number 

 of lines treated for those spectra is sufficient to show clearly the degree of correspondence. 



