ABSOEPTION OF LIGHT IN GASEOUS MEDIA. 
809 
radiation.'^ In tlie present paper no account is taken of tlie reHecting power of tlie 
earth’s surface, d’he tolerable agreement between the results of observation and 
those of a theory leased on a non-retlecting surface shows that for an ordinary laiid- 
scape the effect of reflection from fleld and foliage need not l)e as great as is sometimes 
supposed.t The effect of snow on the polarization of sky radiation is well-knownj : 
on this point Lord Rayleigh mentions in his 1871 paper the interest wliich would 
be attached to sky radiation observations taken over a landscape covered with snow 
or over the sea, reflection from these surfaces being in both cases especially 
determinate. § 
For meteorological purposes it is important to know the total solar radiation 
incident on a horizontal plane as well as the contribution due to sky radiation. The 
results are given in calories per square centimetre per minute for Mount V/ilson and 
Washington in Tables YII. and IX., and are shown graphically in Diagrams IX. and X. 
It will be noticed that for large zenith distances of the sun the contribution of sky 
radiation to the total radiation on a horizontal plane is a very considerable fraction of 
that due to direct solar radiation. The factor of sky radiation is thus of considerable 
importance in the meteorology of northern latitudes. 
The agreement lietween the results of calculations and such o1)servations as are 
available gives rise to a hope that the present communication may serve as a guide 
towai’ds systematic observations of the type dealt with, and to their interpretation in 
terms of a theory of scattering and absorption ; by tliis means one may hope to obtain 
al)Sorption constants and methods of using them which will be of some service to 
meteorology and astrophysics. 
Sumriiavy. 
The analysis of tlie present paper seems to support the view that at levels above 
Mount AVilson molecular scattering is sufficient to account completely both for 
* In the “ Report on the Astrophysical Observatory” (C. G. Abbot, ‘Annual Report of the Smithsonian 
Institution,’ 1911, p. 65), the Director announces that sky radiation observations have been succes.sfully 
taken at Mount Whitney (August, 1910). Since the transmission coefficients at the time of ol)scrvation 
are also determined, the results will enable an accurate comparison of sky radiation results to be made 
Muth the values obtained by calculation in terms of the coefficients of attenuation. 
t Nicols, “Theories of the Colour of the Sky,” ‘Phys. Rev.,’ XXVI., June, 1908, p. 507. 
I Observations have been made by McConnel, J. C., on the effect of the nature of the ground on the 
degree of polarization of the sky at 90 degrees from the sun ; the effect of a covering of snow is to 
diminish the degree of polarization. (Perntner, ‘Meteorological Optics,’ Part IV., 1910, p. 643. The 
above reference is taken from ‘ A History of the Cavendish Laboratory,’ Longmans, Green & Co., 
1910, p. 129.'-) 
§ The analysis of the present paper could be extended wuthout difficulty to take into account the effect 
of reflection from the earth’s surface on the intensity and polarization of sky radiation, provided the 
landscape were covered by a layer of uniform substance such as snow, or observations were taken over tlie 
sea, which presents a determinate reflecting surface. 
