1919] on Energy Distribution in Spectra 525 



associated with the Doppler effect. The coutoiirs associated with the 

 latter should be thin symmetrical parabolas. Those we found were 

 wedge-shaped, with definite kinks indicating the introduction of new 

 component lines when the condenser was put into the circuit. The 

 wedge-shape indicates that the law of decrease of intensity from the 

 centre of a component is expotential, and not the law of error as in 

 tlie Doppler effect. By measuring the distances between the kinks, 

 and knowing the magnification and a previous calibration of the 

 wedge, all necessary quantitative data of the spectrum line can be 

 calculated. It was possible to show, in particular, that the separa- 

 tions in wave-length of the components of Ha were those found by 

 Stark when new components were called into being by the existence 

 of an enormous external potential gradient. As we had suspected, 

 the origin of this exceptional broadening under the condensed dis- 

 charge is the "electric Zeeman effect," the origin of the large electric 

 field on any atom being the close proximity of other charged atoms. 

 We thus have a new means of studying the electrical resolution of 

 spectrum lines, more convenient in many ways than the older methods, 

 and capable of much greater generality and accuracy. A large 

 number of observations of the same phenomenon were made also on 

 the spectrum lines of helium and lithium, and the correspondence 

 with the Stark effect always held good. 



The examination of an individual line has also been applied in 

 the case of an " ordinary " discharge, and given the first direct proof 

 of the probability distribution of velocities in the radiating atoms of 

 a gas. This distribution has been taken as a basis by Lord Rayleigh 

 in the elaboration of a precise method of determining the mass of a 

 radiating atom from the breadths of the spectrum lines, a method 

 applied by Buisson and Fabry with great success, when the breadth 

 is measured by methods of interference of light. Our experiments 

 have defined very closely the circumstances in which this method is 

 practicable, and shown that it fails altogether if condensed discharges 

 are employed. In the ordinary uncondensed discharge under low 

 pressure, however, our contours are very accurately parabohc, which 

 fact can be shown to imply a very rigorous probability law of veloci- 

 ties in the atoms, and no other important source of broadening of 

 lines in these circumstances. 



The only other application to an individual line which I shall 

 mention concerns the nature of the Balmer series of hydrogen, long 

 believed to be a Diffuse Series, with each line consisting of two close 

 components, hardly separable or not separable at all, with the same 

 interval in frequency between them for every line. We have shown 

 that it is in fact a Principal Series, with the separations decreasing in 

 a calculable way required by theory, conforming also the separation 

 in Ha given by Buisson and Fabry. The method was to use the 

 neutral wedge in combination with another apparatus of extreme 

 resolving power — in this case a Lummer-Gehrcke plate. We can in 



