ABSORPTION OF LIGHT IN GASEOUS MEDIA. 
427 
Diagram VIII .—Comparison of Formulce for the Relative Intensities of Shj Radiation Calculated from, Mount 
JFilson Observations f<yr ^ = 60°. 
It is of some interest to compare with the formula for sky intensity derived in the present paper the 
various formulae which have been employed by various writers in the interpretation of sky observations. 
The results are shown graphically in Diagram VIII. for a zenith distance of the sun of 60°, and refer to 
intensities of zenith sky. 
Curve I. is given by the formula (63) 
(47r)-iS|(l+cos2f)C, 
and is the same as that given by Kelvin* and employed by Perrin! in an estimate of the 
number of molecules per unit volume of a gas. 
Curve II. is given by the formula 
(Itt)-! S {f(l + cos2 ()} c, 
and is seen to give a considerably smaller value of sky intensity in the red, agreeing closely with 
the value T (0, () for wave-lengths greater than GS/x. 
Curve III. is given by the mean solution (64) for T (0, ^) which takes into account the effect of self- 
illumination. 
Curve 1V. gives the intensity of zenith sky neglecting self-illumination but taking into consideration the 
attenuation of both the incident and scattered radiation. 
* Kelvin, ‘Baltimore Lectures’ (1904), p. 31.3. 
t Perrin, J., ‘ Annales de Cliimie et de Physique,’ 8™' Serie, September, 1909. See p. 79 of ‘ Brownian Movement and 
Molecular Reality’ (F. Soddv, 1910), published by Taylor and Francis, London. 
Diagram VIII. Comparison of formulae for intensities from zenith sky, Mount Wilson, ( = 60°. 
3 I 2 
