The Astrophysical Observatory of the 
Smithsonian Institution * 
By CG. C. Assot, Research Associate, Smithsonian Institution 
[With 6 plates} 
The Astrophysical Observatory of 
the Smithsonian Institution is the 
child of S. P. Langley, third Secretary 
of the Institution. Owing to his very 
keen interest in the sun’s radiation, he 
had invented the bolometer? about 
1878, and about 1890 equipped it with 
photographic registration. His studies 
of the solar and lunar energy spectra 
were made not only at the Allegheny 
Observatory, but also at the summit 
of Mount Whitney, Calif., where he 
conducted an expedition to measure 
the amount of solar radiation. 
As Langley had been so exclusively 
occupied with the distribution of 
energy in spectra, it is a little strange 
that in the first years of the Astro- 
physical Observatory at Washington, 
1890-1900, he turned aside to the 
relatively less interesting subject of 
mapping the positions of lines in the 
infrared spectrum of the sun. Yet 
that was the Observatory program 
when I joined its staff in June 1895. 
In order to observe infrared radia- 
tion not transmitted by glass, we were 
using large rock-salt prisms made from 
clear blocks of salt obtained from the 
Russian Government. Two noble 
rock-salt prisms were in our equip- 
ment, each about 7 inches tall and 
about 5 inches on edge. With these, 
after improving the bolometer so that 
1 Reprinted by permission from Leaflet No. 
216, Astronomical Society of the Pacific, 
with revision and the addition of illustrations 
by the author. 
2 The bolometer is an exceedingly sensitive 
instrument for measuring radiation by 
recording changes in its electrical resistance, 
produced by heating. 
817869—49—_12 
drift and wiggle in the photographic 
trace of its galvanometer were greatly 
reduced, F. E. Fowle and I mapped 
the solar spectrum to wave length 5.3 
microns, 53,000 angstroms, observing 
indications of more than 600 spectral 
lines and bands of solar or terrestrial 
origin. It was necessary to determine 
more exactly the dispersion of rock 
salt in order to fix the wave lengths. 
Our photographic registration, with 
the highly accurate mechanism fur- 
nished by Warner and Swasey, enabled 
us to give the values of the index of 
refraction of rock salt to six places of 
decimals. Because European observers 
scoffed at such pretensions, and cut 
our values down to four places, it 
was a comfort to us when Paschen 
repeated the determination and, find- 
ing only small differences from us in 
the sixth place, referred to our work 
as “‘of wonderful precision.” 
At the request of H. D. Babcock, 
and with the aid of Hugh Freeman, I 
repeated the bolometric mapping of 
the infrared solar spectrum in 1928 
on Mount Wilson. With higher dis- 
persion, improved apparatus, and 
long bolometric experience, we were 
able to detect evidences of over 1,200 
lines and bands, some of solar, others 
of terrestrial origin, between wave 
lengths 0.76 and 2.0 microns. It was 
a great satisfaction, when I discussed 
this work with Babcock in 1945, to 
learn that he finds it still useful, and 
that there is almost nothing in our 
curves that is not confirmed by present 
photographic spectra or by theoretical 
physics. 
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