70 



ANNUAL EEPOET SMITHSONIAN INSTITUTION^ 1908. 



The observations. — The approach of totality was uncommonly exciting on this 

 occasion. Early in the morning the sky was overcast with thin high clouds, but 

 these gradually grew thinner, so that after 9 a. m, the prospects indicated a 

 streaky sky, containing something almost too thick for haze, but almost too 

 thin for cirrus clouds. These prospects were fulfilled exactly during totality, 

 but in the quarter of an hour next preceding a thick cloud came up, rain fell 

 fast from 11 h. 8 m. to 11 h. 14 m., and the sky became clear of the low cloud 

 only fifteen seconds before totality at the Smithsonian station. The rapid 

 change from fair prospects to completely discouraging ones, and the return to 

 good conditions just at the critical time, will long be remembered. Our entire 

 immunity from rain during totality was due to the fact that our station was 

 about 1,000 feet north of the one occupied by the Lick Observatory. 



The intensity and quality of sunlight was determined within twenty-five min- 

 utes of totality, both before and after, and during totality measurements were 

 made at five difi'erent regions of the corona and on the dark moon. A general 

 summary of the results of these and other observations follows: 



Intensity of rays {observed through glass). 



Intensity for unit 

 Source. angular area. 



Sun near zenith, Flint Island 10,000,000 



Sky 20° from sun, Flint Island - 140 



Sky far from sun, Flint Island 31 



Sky average, Flint Island 62 



Sky average, Mount Wilson. Cal 15 



Moon at night, Flint Island 12(?) 



Moon during eclipse, Flint Island 



Corona tV radius froiu sun 13 



Corona \ radius from sun 4 



Corona f radius from sun 



Proportion of rays irhieh asphaltum transmits. 



Source. 



Determination. 



Mean 

 (weighted). 



Sun J5 radius from limb 



Corona yV radius Irom limb. 

 Corona J radius from limb.. 



Moon at night 



Sky, zenith day 



0. 333 

 .343 



.387 



0.331 



.484 



a. 323 



0^332 

 .364 

 .362 

 .5 

 .23 



■ This observation is entitled to only half the weight of the others. 



Discussion of the results. 



When we recall the extreme brightness of the sky within a single degree of 

 the sun, as compared with that 20° away, and consider also the figures just 

 given, it seems very unlikely that the corona will ever be observed without an 

 eclipse. 



The nature of the radiation of the inner corona has been supposed by some 

 to be principally refiected solar radiation, by others to be principally due to 

 the incandescence of particles heated by reason of their proximity to the sun, 

 by others to be principally luminescence perhaps similar to the aurora, and by 

 some as a combination of all of these kinds of radiation. 



The spectrum of the corona is mainly continuous, but has some inconspicuous 

 bright lines, and in its outer part has dark solar lines. Undoubtedly there is 

 sunlight reflected by the matter of the corona, .and no less surely the corona 



