OF THE 1918 ECLIPSE. 271 



Our comparison of the corona with the sky before and during 

 the ecHpse shows that, in terms of a circle of sky of the apparent 

 size of the sun^, the corona gave one tenth as much hght in full sun- 

 shine, but more than 600 times as much during totality. The de- 

 crease in the light of the sky due to the moon's shadow was there- 

 fore 6,000-fold. As the decrease from sunlight to corona light is 

 fully 100 times 6,000, this means that not more than i per cent, of 

 the general sky illumination during an eclipse can come from the 

 corona, the remainder being from sunlight reflected from the earth's 

 surface and atmosphere which is outside the moon's shadow. 



The result that the corona gives one tenth of the light from a 

 circle of daylight sky of the same area as the sun, and 8° away, has 

 a direct bearing upon the problem of detecting the corona without 

 an eclipse. Experiments in using photo-electric cells for this pur- 

 pose have already been begun by Dr. Hale at the Mt. Wilson 

 Observatory. 



University of Illinois, 

 Observatory. 



