ASTRONOMY: R. W. WOOD 
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appears to show that the phenomenon is real. This pecuharity was, 
however, not detected until the work was finished, and I should prefer 
to verify it, or have it verified, before setting it down as an established 
fact. The luminosity is much too feeble to show in the prints. Dr. 
Hale will have the density measured with the Hartman photometer 
as soon as some repairs are finished. I do not, however, believe that 
the dark belt is in reality due to the absorption of a dust ring, for measure- 
ments made by Mr. Ellerman showed that the belt extended higher up 
on the ball of the planet than the line of intersection of the plane of the 
rings. Moreover it seems highly probable that the belt and the dark 
polar cap are to be explained in the same way. The second hypothesis 
assumes the existence in the planet's atmosphere of some substance 
capable of absorbing violet and ultra-violet light. This material might 
be a fine mist or dust, or some gas capable of absorbing the more re- 
frangible part of the spectrum. Such a gas would be of a pale yellow 
color, and sulphur vapor and chlorine naturally occur to us. 
I have, however examined the absorption spectrum of both of these 
gases in quartz bulbs, with the result that the absorption appears to be 
much stronger in the ultra-violet than in the violet, which is in dis- 
agreement with the circumstance that the band appears wider in violet 
than in ultra-violet light. Until spectrograms are available showing 
the complete spectra of the different zones of the planet it is useless 
to speculate as to the nature of the material which causes the absorption. 
On the infra-red photograph the ball of the planet is much brighter in 
y£.LLov/ 
ULTfKA- \/!oL£t, 
MONOCHROMATIC PHOTOGRAPHS OF SATURN, OCTOBER 23, 1915 
