SOLAR ECLIPSE OF JANUARY 3, I908 — ABBOT 39 



good blue color for a considerable time, numerous measurements 

 were made with pointings on the center of the sun's disk and on 

 many parts of the sky. In these measurements the full aperture of 

 the concave mirror was employed for the sky, and the "0.316" 

 aperture for the sun. Sun-brightness was further reduced by inter- 

 posing in the beam a rotating disk from which a sector of -045 of the 

 whole circle had been cut. Eight ohms' resistance was placed in 

 series with the galvanometer, under which circumstances i ohm 

 change in the balancing resistance produced 55 mm. deflection. The 

 measurements were begun about 9^^ 40"^ a. m. and continued till 

 10^ 40™ a. m., local time, so that the sun was 20° to 40° from the 

 zenith. 



Reserving for another publication a detailed study of these and 

 other comparisons of sun and sky brightness, it will be sufficient 

 here to state that the relative brightness of sky and sun, equal areas 

 being measured, varied from 0.0000031 at distant parts of the sky to 

 0.0000140 at 20° from the sun. The average value was 0.0000062. 

 It was impossible to secure accurate observations nearer the sun than 

 20°, because the mirror could not be properly shaded from the sun 

 in such cases, and the dififused reflection of sun-brightness would 

 have masked the true sky-brightness. 



Measurements made on Mount Wilson, in California,^ in 1905-6 

 showed that the average ratio at that altitude was about 0.0000015, 

 so that the sky at sea-level appears to be, roughly, four times as 

 bright as on Mount Wilson. 



From measurements made on December 2y at 3^ 30™ a. m., the 

 moon-brightness was about 0.0000012 of sun-brightness; but this 

 ratio can only be regarded as roughly approximate,- and likely to be 

 altered with the haziness or humidity of the air as well as with the 

 altitudes of the sun and moon. 



Quality of Sun-brightness, Sky-brightness, and Moon-brightness 



On December 26, with the sun about 40° from the zenith, Ihe ratio 

 of the sun-brightness transmissible by the asphaltum screen to the 

 total sun-brightness was found to be 0.366, while for zenith sky- 

 brightness the result came out 0.248. Owing to the change of hu- 

 midity from time to time, with consequent large alterations of the 



^ Altitude, 1,800 meters. 



^ This ratio is not directly comparable with the determinations which dif- 

 ferent observers have made of the relative photometric measures of the light 

 of the sun and moon, nor, on the other hand, with determinations of the 

 relative amounts of the total radiation of the sun and moon. 



