154 DK. T. K. MEETON AND PKOF. J. W. NICHOLSON ON 



that its intensity on the true scale does not exceed unity, and therefore on the present 

 scale does not exceed 10/G'25 or T6. Comparison with the remainder of the table 

 shows therefore that its absence is to be expected if the drop of intensity from I. to 

 IT. is not of a different order from that found in any other series. The disappearance 

 of the same line from IV. and later photographs is also to be expected, unless a great 

 increase of its relative intensity takes place suddenly at this point. 



In the case of Parhelium, the Sharp and Principal series decrease in intensity as 

 compared with the Diffuse series, without the temporary reversal of this phenomenon, 

 at the end of the dark space, found in the case of Helium. The apparent reversal at 

 a considerable distance, found in the case of Helium on one plate of the Principal 

 series and stated not to be decisive, is repeated on two plates in the Principal' series 

 of Parhelium (VI., VII.) and now appears to be real. Very considerable exposures, 

 however, would be necessary at greater distances in order to establish the fact that 

 the phenomenon continued to occur. We have preferred, in the experiments recorded 

 in this communication, to confine attention to a series of photographs taken with 

 identical duration of exposure. 



It is difficult to draw any conclusions, in the case of Parhelium, with regard to the 

 relative transfer of eneigy between the Sharp and Principal series, for the former is 

 only visible on two photographs. The only definite difference of behaviour with 

 regard- to Helium and Parhelium thus appears to lie in the region at the end of the 

 dark space, where there is a temporary relative diminution of the Diffuse series of 

 Helium, but not of Parhelium. 



(VII.) Comparison of Helium and J'tir/ieliiim. 



A related problem of some interest is the determination, on some precise basis, of 

 the relative changes which take place in the corresponding doublets (Helium) and 

 single lines (Parhelium) in the spectrum. We have seen in the last Section that the 

 relative phenomena of the three series are the same in general in each case, except 

 for a small difference on photograph II. The best standard of comparison is 

 apparently given by the leading lines of the three series in each case. 



We accordingly compare XX587G, 6678 as the leading lines of the two Diffuse series, 

 XX4713, 5047, for the Sharp series, and XX3888, 5015, for the Principal series. 

 Intensities in the doublet series are all reduced to 10. 



The disappearance of 6678 on VIII. is in accordance with a still further reduction 

 of its intensity on this scale, below 2 '05, so that the decrease of relative intensity 

 persists to the extreme photograph. There is a reversal at the end of the dark space 

 on II. in the usual manner, the conditions of emission in this region evidently 

 possessing special features which affect all the lines in the spectrum. Apart from 

 this effect, the Parhelium Diffuse series steadily decreases in intensity, with increase 

 of distance from the cathode, relatively to the Helium Diffuse series. The phenomenon 



