64 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 



day in its output of radiation within limts of from 5 to 10 per cent in quantity 

 and iu 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. 



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 Moxmt 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 brlghtness=l. Mount Whitney, 1909-1910.1 



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. 



