Radiation and the Structure of the Atom. 40$' 



out in paper IV. that the deviation of the function <j>(n) 

 from unity gives us an estimate for the deviation of the 

 forces from the inverse square, and that on the theory we 

 can only expect a Stark effect of the same order of magni- 

 tude as for the hydrogen lines for those series in which <£. 

 differs very little from unity. 



This conclusion was consistent with Stark's original 

 measurements of the electric effect on the different series in 

 the helium spectrum, and it has since been found to be 

 in complete agreement with the later measurements for a 

 great number of other spectral series. An electric effect of 

 the same order of magnitude as that for hydrogen lines has 

 been observed only for the lines in the two diffuse series of 

 the helium spectrum and the diffuse series of lithium. This 

 corresponds to the observation that for these three series cj> is 

 very much nearer to unity than for any other series ; even for 

 n = 5 the deviation of </> from 1 is less than one part in a 

 thousand. The distance between the outer components for 

 all three series is smaller than that observed for the hydrogen 

 line corresponding to the same value of n, but the ratio 

 between this distance and that of the hydrogen lines ap- 

 proaches rapidly to unity as n increases. This is just what 

 would be expected on the above considerations. The series 

 for which the effect, although much smaller, comes next in 

 magnitude to the three series mentioned, is the' principal 

 single line series in the helium spectrum. This corresponds 

 to the fact that the deviation of (/> from unity, although 

 several times greater than for the three first series, is much 

 smaller for this series than for any other of the series ex- 

 amined by Stark. For all the other series the effect was 

 very small, and in most cases even difficult of detection. 



Quite apart from the question of the detailed theoretical 

 interpretation of the formula (13), it seems that it may be 

 possible to test the validity of this formula by direct 

 measurements of the minimum voltages necessary to produce 

 spectral lines. Such measurements have recently been made 

 by Rau * for the lines in the ordinary helium spectrum. 

 This author found that the different lines within each series 

 appeared for slightly different voltages, higher voltages 

 being necessary to produce the lines corresponding to higher 

 values of n, and he pointed out that the differences between 

 the voltages observed were of the magnitude to be ex- 

 pected from the differences in the energies of the different 

 stationary states calculated by (13). In addition Rau found 

 that the lines corresponding to high values of n appeared for 

 * Rau, loc. cit. 



