400 
ME. J. EYERSHED ON AY AYE-LENGTH DETERMINATIONS, ETC., 
Helium and Hydrogen in the Lower Chromosphere. 
The part played by helium and hydrogen in the lower chromosphere may next he 
considered. Helium is probably absent from the very lov'est strata, if indeed it is 
present at all in the flash-spectrum layer. The lines at XX 3634‘4, 4026’3, 4471’6, 
emd 4713’2 are very weak, or quite invisible upon the continuous spectrum at the 
centre of the photograph (No. 3) although strong on either side, whilst the majority 
of the lines of the flash spectrum proper are strongly impressed right across the con¬ 
tinuous spectrum. This woidxl point to the absence of helium from the region within 
2" of the photosphere. 
[But the lines at XX 4922T and 5015'7 appear on the other hand to be strong on 
the continuous spectrum and weak at the sides. It is to be remarked, however, that 
ihese lijies belong to two series of lines in the spectrum of cleveite gas which have 
been ascribed to parhelium by Plunge and, Paschex."^^ It would seem, therefore, 
that these “ parhelium ” lines, whatever their origin, are produced under conditions 
existing only in the lower levels.—ilTcrrc^ 10, 1901.] 
The absence of appreciable absorption due to helium in the solar spectrum seems also 
to favour the view that this element exists only in a rarefled condition in the upper 
cliromosphere. Tliis peculiarity in the spectrum certainly does not necessarily imply 
equality of temperature between the radiating gas and the })hotospheric background ; 
for the hydrogen spectrum affords a demonstration that chromospheric gases at a 
lower temperature tlian the photosphere, can emit strong bright lines without corre¬ 
sponding absorption lines. Thus, in the visible spectrum the dark lines corresponding 
to Ha,- y, and 8 are suflicient evidence that hydrogen in the chromosphere is cooler 
than the photosphere. But in the ultra-violet the absorption becomes inappreciable 
at H{, and beyond this point there are no dark hydrogen lines corresponding with 
the strong bright lines in this region. 
In the emissioir sj^ectrum of the chromosphere there is a progressive diminution of 
intensity of the hydrogen lines towards the smaller wave-lengths. Nevertheless, the 
lines Hy, d, i, k are still among the strongest lines in the spectrum beyond K. In the 
ultra-violet, therefore, hydrogen behaves exactly like helium in the visible spectrum. 
The disappearance of the hydrogen absorption lines in the ultra-violet has been 
ascribed by PtOAVLAND to excessive difiuseness or Avidening of these lines, and if 
this AoeAV is correct it follows, as Sir William Huggixs has pointed out, “that the 
hydrogen absorption in the sun must be restricted to a narroAv region Ioav down and 
close upon the photosphere itself,”! the hydrogen in the higher regions contributing 
little or nothing to the absorption. 
This vieAv, hoAveAmr, AAliich seems to imply the complete independence betAveen 
* Runc 4E and Pasciiex, “ On the Spectrum of Cleveite Gas,” 
(1896). 
t ‘ An Atlas of Rejn’esentative Stellar Spectra,’ p. 150. 
Astro.Thysical Journal,’ vol. 3, No. 1 
