170 
With present knowledge we feel 
sure that the supposed change of 10 
percent in the solar constant in March 
1903 was illusory. At that time we 
had not perfected our methods to 
avoid errors in estimating the at- 
mospheric losses. About that time, 
as in the eruption of Mount Katmai, 
Alaska, in 1912, there were several 
violent volcanic eruptions, including 
Colima, Mexico, and Pelee, West 
Indies. These, as in 1912, probably 
caused a great change in the at- 
mospheric transparency. This would 
alter the temperature of the North 
Temperate Zone, and at the same 
time alter the error of our imperfect 
solar-constant determinations. Never- 
theless, erroneous as was our impres- 
sion, it caused us to undertake the 
long program of observing the varia- 
tion of the sun which still goes on, 
and which has produced important 
results. 
When the Mount Wilson Observ- 
atory was established in 1904, by the 
efforts of George E. Hale, Langley 
was invited to use that station for 
determinations of the solar constant of 
radiation and the range of variation 
of the sun’s output of radiation. In 
1905 I went to Mount Wilson with a 
large outfit, and was joined by L. R. 
Ingersoll, now head of the department 
of physics, University of Wisconsin. 
From 1905 to 1922, with few excep- 
tions, my colleagues and I spent 
several months each year in solar- 
constant measurements on Mount 
Wilson. In more recent years I have 
occasionally revisited Mount Wilson 
for special observations. 
Thus I have been priviledged to 
watch, as with a bird’s-eye view, those 
great advances in astronomy which 
have justly given Mount Wilson 
Observatory a shining place in the 
world. I like to recall from time to 
time some of them. Hale’s discovery 
of magnetism in sunspots; Adams’ 
and St. John’s spectroscopic measures 
of rotation of the sun; St. John’s 
exact spectrum places; Adams’ and 
ANNUAL REPORT SMITHSONIAN INSTITUTION, 1948 
Kohlschutter’s spectroscopic paral- 
laxes; Nicholson’s and Pettit’s meas- 
ures of stellar and planetary radiation; 
Hubble’s and Humason’s work on 
the distant nebulae; Seares’ photo- 
metric studies; and many more fine 
pieces of work that it is invidious not 
to mention. I saw the mountain 
transformed by George Jones’ years 
of effort; the building of the road; 
the erection of the two solar tower 
telescopes and of the 60-inch and the 
100-inch reflectors. I saw G. W. 
Ritchey’s beautiful direct photographs 
with the 60-inch; the long spectro- 
scopic programs which have revealed 
so much about the stars; especially 
the great detailed stellar spectra by 
the coudé spectroscope of the 100- 
inch. All these wonderful opportuni- 
ties of seeing astronomy in the making, 
and of knowing the fine men and 
women of the staff of Mouat Wilson 
Observatory, have indeed been a 
profound satisfaction in my life. 
In the many years during which 
my colleagues and I regularly re- 
turned to Mount Wilson, we made 
many observations, in addition to 
those of the solar constant. As our 
measurements seemed to indicate a 
variability in the sun’s output of 
radiation, we sought to check this 
important possibility in several ways. 
We erected a tower telescope on our 
observatory, and every day, after 
observing for the solar constant, we 
drifted an 8-inch image of the sun 
over the spectrobolometer, selecting 
a number of different wave lengths 
These drifts gave U-shaped curve. 
for the sun is less bright near the 
limbs than at the center. The con- 
trast is much greater for violet than 
for infrared rays. It was our object to 
detect whether variations of the solar 
constant were associated with varia- 
tions of the contrast of brightness 
between the edge and center of the 
sun. Some indications of such a cor- 
relation were indeed found. Another 
test of our supposed solar variation 
we made by expeditions to the summit 
