154 
DR. T. R. MERTON AND PROF. J. W. NICHOLSON ON 
that its intensity on the true scale does not exceed unity, and therefore on the present 
scale does not exceed 10/6’25 or 1'6. Comparison with the remainder of the table 
shows therefore that its absence is to be expected if the drop of intensity from I. to 
TT. 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 (VL, VH.) and now appears to be real. Very considerable exposures, 
however, would be necessary at greater distances in order to establish the fact that 
tlie 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 twm 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 Parlhelium. 
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 H. The best standard of comparison is 
apparently given by the leading lines of tlie three series in each case. 
We accordingly compare XA587(3, 6678 as the leading lines of the two Diffuse series, 
AA4713, 5047, for the Sharp series, and AA3888, 5015, for the Principal series. 
Intensities in the doublet series are all reduced to 10. 
Tlie disappearance of 6678 on VHI. 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 
