SOME PROPERTIES OF LIGHT OF VERY SHORT WAVE LENGTHS. 133 
tails stretch toward the red. This spectrum resembles the fourth group 
of Deslandres,! and appears to form a continuation of it. The bands 
extend from the region \ 2600 to X 1300; they afford a wide field for 
a test of Deslandres’ law. 
The spectrum of argon consists of a considerable number of 
characteristic lines extending from A 2000 to the limit of the spectrum. 
Up to the present time it has been impossible to attribute any lines 
to helium, to oxygen, or to nitrogen, in that part of the spectrum more 
refrangible than 1850. 
A great number of solids have been examined, with the hope of 
finding some substance more transparent than white fluorite.? The 
results have been negative. Rock salt in thicknesses of one or two mil- 
limetres begins to absorb strongly in the neighbourhood of A 1750. 
B.O; is even less transparent.* Quartz in thickness of 2 mm. absorbs 
strongly at X 1500, and the absorption increases rapidly with thickness ; 
even white fluorite becomes opaque near A 1250. It was the opacity of 
fluorite which set the limit to Schumann’s progress in the extreme 
ultra-violet. It was by the removal of all fluorite from the light path 
and by the use of a grating, that the writer has been able to extend the 
spectrum to its present limit—ad 1030. 
It is interesting to note that the positions of the absorption bands 
demanded by Maclaurin’s * dispersion formula for rock salt and fluorite 
lie in the neighbourhood of A 1265 and A846 respectively ; while on the 
experimental side strong absorption appears to begin near » 1750 for 
rock salt and near X 1250 for fluorite. 
The investigation of the spark or arc spectra of solids has met with 
very little success. Apart from a few lines in aluminium extending into 
the neighbourhood of A 1650 almost nothing has been accomplished.°® 
The difficulties are obvious. It is necessary to produce a spark or arc 
in such a manner that the light characteristic of the substance in question 
may enter the vacuum spectroscope without suffering absorption. The 
high potential arc * has been tried in this connection, but without result. 
On the photo-chemical side of the subject the well-known action of 
light in producing ozone is perhaps the most conspicuous phenomenon. 
The fact of importance to be deduced from a study of the Schumann 
region relates to the rapid increase of this effect with decrease in wave 
length. The very wave lengths which are most strongly absorbed by 
the air are those which are most active in the production of ozone.’ It 
seems probable, therefore, that the strong absorption of oxygen is con- 
nected intimately with this photo-chemical action. 
The formation of ozone may play a considerable part in determining 
the relative velocity of the ions produced by ultra-violet light. 
In photo-electric phenomena it is well known that for many sub- 
stances the discharge of negative electricity becomes more pronounced as 
the wave length of the exciting light is decreased. In the Schumann 
region this effect becomes very striking, as the following experiment will 
serve to indicate. 
1 Compt. rend., V. cvi., 1888, p. 845. * Astrophysical Journal, vol. xxv., No. 1. 
3 Tbid., vol. xxviii. No. 1. 4 Proc. Roy. Soc., A, vol. 81, p. 367. 
5 Kayser, Handbuch, vol. iii. p. 339. 6 Physical Revien, vol. v. p. 1. 
7 Astrophysical Journal, vol. xxvii. No. 2, p. 98.| 
