ASTRONOMY: R. W. WOOD 
109 
MONOCHROMATIC PHOTOGRAPHY OF JUPITER AND 
SATURN 
By R. W. Wood 
DEPARTMENT OF PHYSICS. JOHNS HOPKINS UNIVERSITY 
Received by the Academy, January 28, |916 
Photographs of the moon, which I made several years ago by means of 
ultra-violet light, revealed the existence of an extensive dark area sur- 
rounding the crater Aristarchus, which did not appear on plates made 
by means of yellow light, and could not be detected visually. Control 
experiments, made in the laboratory, made it appear probable that 
this deposit consisted of sulphur or sulphur bearing rock. 
It appeared to me probable that even more interesting results would 
be obtained in the case of the planets, especially Mars, and preliminary 
experiments were carried on during the past summer at my East Hamp- 
ton Laboratory with a horizontal telescope of 56 feet focal length and 
16 inches aperture, figured by Mellish, and nickel plated by the method 
described in the Astrophy steal Journal for October. 
A deposit of nickel is necessary for the reason that silver reflects only 
4% of the light in the region of the spectrum utihzed, which hes between 
wave-lengths 3000 and 3300. This mirror was utilized in conjunction 
with a large Gaertner coelostat, which was placed at my disposal by 
the Naval Observatory. The mirror of this instrument was replaced 
by a 16-inch flat, also nickel plated, and photographs were made of the 
moon and Jupiter by means of infra-red, yellow, violet and ultra-violet 
light. Notwithstanding the fact that a moving plate holder, provided 
with eye-pieces for accurate following, was used, it was found difficult 
to secure sufficiently sharp dehnition, owing to the rather rapid periodic 
drift of the image which was very difficult to follow. The seeing more- 
over was not very good, and on the few nights when it was fair the 
mosquitoes were bad. Much valuable experience was gained however 
in the use of the monochromatic ray Alters. 
In the earlier experiments on the moon, I used, for the ultra-violet 
ray filter, a rather thick deposit of metallic silver on uviol glass. This 
was abandoned in the present work, owing to its great opacity even 
for the ultra-violet region transmitted, and a rectangular glass cell, 
closed with plates of uviol glass, and filled with dense bromine vapor, 
used in its place. Such a cell, when used with an ordinary (i.e., not 
isochromatic) plate, gives a photographic image formed by wave-lengths 
below 3500 exclusively. By combining it with a thin layer of a very 
dilute solution of chromate of potash, the transmission is practically 
