[KING] ABSORPTION OF LIGHT IN GASEOUS MEDIA 137 


Station No 
Potsdamer... 0 ZEN P< Une 
Washington..... 2-24 
Mt. Wilson...... 2-28 
Mt. Whitney... 2-26 

These values are in fair agreement with the values obtained by Ruther- 
ford and Geiger! (2-72 X 10") and more recently by Millikan? (2-644 X 
OS): 
Another numerical result of some interest is obtained from the 
observations for comparatively dust-free air above Mt. Wilson and Mt. 
Whitney. The value of the constant y — a, H is -032 giving a, = 
a0 1052 cm —. 
a, represents the proportion of radiant energy travelling through 
air at 0°C and 760 mm. pressure which is converted into thermal mole- 
cular agitation per centimetre of path. The effect of “dust” e.g. at 
Washington is to increase this value six to eight fold. 
If direct sunlight is travelling through air at normal pressure and 
temperature the value of &, leads to an increase of air temperature of 
-015° C. per hour. 
6. The intensity of sky-radiation can be calculated in terms of 
the attenuation coefficients: the results for zenith sky are obtained at 
Mt. Wilson for comparison with observations and at Washington as 
typical of a sea-level station. The former results agree sufficiently 
well with experiment both as regards quality and total intensity of 
sky-radiation and so indicate that molecular scattering when taken n 
conjunction with self-illumination is sufficient to account for all the 
phenomena of sky-radiation. The usually neglected factor of self- 
illumination is responsible for some 33% of the whole effect at -410 4 
in the violet and for some 5% at -741/ in the extreme red of the spec- 
trum. 
Sky intensities at sea-level are interesting in enabling us to calculate 
the total radiation from the sky on a horizontal surface, a factor of 
some importance in meteorology. The results are summarized in the 
short table given below, calculated on a basisof the mean attenuation 
coefficients for Washington. 

1 Rutherford and Geiger, Proc. Roy. Soc. A, Vol. 81, 1908. 
" Millikan, Phys. Rev. XXXII, April, 1911. 
Sec. III., 1912. 10. 
