INTENSITY RELATIONS IN THE SPECTRUM OF HELIUM. 



155 



TABLE XI. -Helium and Parhelium. 



of the dark space is shown definitely also in the Principal series of Parhelium, which 

 at this point becomes almost as intense as that of Helium. Subsequently the decrease 

 of the Parhelium spectrum is shown definitely until we arrive at VI., where another 

 temporary reversal occurs. Although the photographs VI.-VIII. may not be strictly 

 comparable with the others, the phenomenon appears to be real, for it corresponds to 

 similar effects in previous comparisons of series made in this communication, which 

 are not restricted to special ranges of wave-length in which the plate on which 

 VI.-VIII. were taken might have special properties. Moreover, it does not occur at 

 all in other series, for example the two Diffuse series of the present section, where 

 there is no sudden change in the character of the numbers characterizing X6678 on 

 passing from V.-VI. 



It seems necessary to conclude that there is a region, distant about 5 mm. from the 

 cathode in the present experiment, where, as at the extremity of the dark space 

 1 mm. from the cathode the conditions of excitation reach some form of critical 

 point, with a consequent change in the nature of the law of intensity variation of 

 certain lines and series with cathode distance. In particular, there is a tendency for 

 relative enhancement of the Principal series of Parhelium, but not the Diffuse series, 

 at this point. 



(VIII.) Regions of Maximum Emission. 



The regions in the tube from which individual spectral lines are radiated with 

 greatest intensity are of considerable importance in connection with theories of the 

 origin of spectra. The present measurements enable us to obtain some quantitative 

 data with regard to many lines in the spectrum of Helium. We do not attempt to 

 discuss all the lines from this point of view, the exposure being in many cases only 

 sufficient to show some of the lines on one or two photographs, so that no graphical or 

 other method can be used to determine the exact law which connects their intensities 

 with the distances from the cathode, enabling the positions of the maxima to be read 

 .off the curve of intensity or calculated by analysis. Moreover, it is sufficient, for a 



z 2 



