64 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
day in its output of radiation within limts of from 5 to 10 per cent in quantity 
and in irregular periods of from 5 to 10 days. This conclusion I state tenta- 
tively. Before it can be accepted without question it must be confirmed by 
showing that the results obtained day after day at another equally good station, 
at a great distance, confirm those obtained simultaneously at Mount Wilson. 
Such a final test, it is now expected, will be made during the coming fiscal year. 
Summary of solar-constant values. 
ae Mount Wilson. Mount Whitney. 
1902-1907 1905 1906 1908 1909 1910 1909 1910 
Times observed...... 44 59 62 113 95 128 1 3 
Mean value.........- 1.960 1.925 1.921 1.929 1. 896 1.914 1.959 1.956 
1 Other days of observation not yet ready. 
General mean, 1.922 calories (15° C.) per square centimeter per minute. 
Number of determinations, 405. 
Other observations made on Mount Whitney.—Although the main purpose 
of the Mount Whitney expedition of 1910 was served by proving that the 
determinations of the solar constant of radiation are independent of the altitude 
of the observing station, advantage was taken of the unusual opportunity to 
make several other kinds of observations. Kapteyn’s sky photometer was 
employed there on two successive nights to measure the relative brightness of 
the different regions of the night sky and to estimate the total quantity of sky 
illumination per square degree compared with that of a first-magnitude star. 
Yntema had employed similar apparatus in Holland. He found the average 
brightness of the Milky Way about two or three times that of nongalactic 
regions of the sky, such as the north polar region, but that the sky near the 
horizon was of about the same brightness as the Milky Way. He concluded 
that the sky at night is illuminated more by some terrestrial sources of light 
than by the stars. 
’ The results obtained on Mount Whitney at nearly 3 miles elevation agreed 
in general with those of Yntema. The following is a summary of the prin- 
cipal points. Mean values are given: 
Brightness of night sky. 
[Polar brightness=1. Mount Whitney, 1909-1910.] 
Galactic latitude. 
Near hori- 
zon. 
Peay wR | [ 
0° to+5° SEAS OE a oy core 
Relative brightness.........-..--.--.-. 2.10 1.'25 1.19 1.17 1.40 
The total illumination from 1 square degree of polar sky was found to be 
0.0746 that of one first-magnitude star in the zenith. It is possible that the 
fraction just given may be a little too small, owing to a source of error discov- 
ered after the observations were ended. 
