Voi,. 6, 1920 
ASTRONOMY: C. G. ABBOT 
91 
probably ozone, although much less plentiful in the atmosphere, and cer- 
tainly not more powerful as an absorber for the spectrum of a perfect 
radiator than carbon dioxide, is yet entitled to be regarded as second in 
importance on account of this peculiar posture of affairs. 
3. Studies of Terrestrial Radiation and Its Atmospheric Absorption. — 
The study of these absorbing elements and their influence upon the spec- 
trum has been carried on to a considerable extent by several investigators, 
notably by Paschen, Rubens and his collaborators. Miss Eva von Bahr, 
and lately by Mr. Fowle of the Astrophysical Observatory of the Smith- 
sonian Institution. Mr. Fowle' s researches^ comprised the examination 
of the spectrum of the Nernst Glower up to a wave-length of about 17 
microns, particularly as it is affected by the absorption occurring in long 
columns of atmospheric air containing known amounts of humidity and 
carbon dioxide. The experiments deal with quantities of from 0.003 
to 3.0 centimeters of liquid water disposed in the atmosphere as water' 
vapor. Owing to the lack of a suitable transparent refracting medium 
for optical purposes beyond the wave-length 17 microns, the important 
region of the terrestrial spectrum extending from 17 to 50 microns could 
not be investigated by Mr. Fowle. Somewhat later, Mr. Aldrich of the 
Astrophysical Observatory of the Smithsonian Institution made a long 
series of experiments (as yet unpublished) with crystals both natural and 
artificial and with other available substances, endeavoring to discover 
one which would be suitable for making a prism transparent to these 
rays. The only substance found which seemed to be notably superior 
to rock salt is potassium iodide, and this material has hitherto been avail- 
able only in very minute crystals. Promising experiments made at the 
General Electric Company indicate that large crystals of potassium 
iodide for prisms may soon be available. If so, it will be possible to carry 
on this valuable spectrum investigation over a region hitherto very little 
explored. The dependence of terrestrial radiation on water vapor has 
been investigated to a considerable extent in another way by Dr. A. K. 
Angstrom, who made observations in Algeria, Southern California, and 
lately in Sweden with the pyrgeometer, a nocturnal radiation measuring 
instrument invented by K. Angstrom. Dr. Angstrom's measurements 
were conducted during several months at a considerable number of sta- 
tions varying in altitude from near sea level to that of Mt. Whitney, 
4,400 meters, and in dryness from desert conditions to those of consider- 
able humidity. At the same time measurements were made of the hu- 
midity prevailing in the air, partly at the surface by means of psychrome- 
ters and partly in the free air by means of kites and sounding balloons 
under the management of the United States Weather Bureau. This 
valuable investigation was supported by the Hodgkins Fund of the Smith- 
sonian Institution, and was pubUshed as Vol. 65, No. 3, of the Smith- 
sonian Miscellaneous Collections. 
