OF ARTS AND SCIENCES. 



425 



In the following table the minutes after sunset, or abscissas, and the 

 percentage of light as compared with sunset, or ordinates, are given, 

 from which the last curve was constructed : — 



TABLE III. 



IV.— LIGHT OF THE SKY. 

 By W. O. Crosby. 



The light of the sky is reflected light, of which the sun is the source. 



It is well known that the light of the sky diminishes as the angular 

 distance from the sun increases. And the following observations were 

 made with a view, first, to determine the absolute amount of light 

 received from the sky at different distances from the sun ; secondly, to 

 ascertain the law of the diminution of the light with increasing angular 

 distance from the sun. The apparatus employed consisted of a com- 

 mon mirror, so arranged as to reflect the light horizontally into a 

 darkened room, a condensing lens having an aperture of 9 cm. and a 

 focal distance of 225 cm., and a photometer similar to that employed 

 by Dr. "Williams in his observations on twilight. 



The method pursued was to so adjust the mirror and lens that an 

 image of the sun would fall upon the disk of the photometer, and then 



