426 
ME. LOUIS VESSOT KING ON THE SCATTERING AND 
Diagrams II.-VII .—Relative Intensities from Zenith Sky Calculated for Different JFave-Lengths at Mount 
JFilson from Mean Coeffi-cients of Attemiation. 
The curves given on Diagrams II.-VII. are calculated for various wave-lengths from formula (64) in 
terms of the mean coefficients of attenuation for Mount Wilson given in Table II. The numerical values 
from which these curves are drawn are given in Table VI. The three outer curves of each set represent 
the extreme and mean solutions of the integral equation (64), 
T (0, 0 = (4:r)-i [f (1 + cos2 0] s [P (0, 0 + Q, (0, 0] . f/C, 
P(0,0 = Ce-CG[C(secC-l)] 
Q(o,f) = i^0ffi(C,o). 
The inmost curve of each set gives the intensity from zenith sky neglecting self-illumination, i.e., it 
represents the value of 
(47r)-i[f(l + cos2 0] SP(0, O-f/C. 
In this way the contribution of self-illumination to the intensity of sky radiation is made clear. We 
notice the divergence of the extreme solutions for large values of the sun’s zenith distance. 
The curves just described resemble the curves obtained by Nicols* * * § in his observations on the intensities 
in the spectrum of zenith sky compared with corresponding intensities in the spectrum of an acetylene 
flame. The results given by Nicols were taken at various times of the day (July 18, 1907) at Sterzing 
in the Tyrol (lat. 46° 54' N., long. 11° 25' E., elevation 3110 feet above sea-level). Numerous observations 
have been taken by Crova,! Zettwuch|, Ma.torana,§ and others.jl The results are not strictly 
comparable with theory since the attenuation coefficients at the time of observation are not given. The 
forthcoming observations of the Smithsonian Astrophysical Observatory on sky radiation at Mount 
Whitneyll (August, 1910) will enable an accurate comparison to be made with the results calculated from 
the attenuation coefficients measured on the same day. 
where 
and 
* Nicoxs, E. L., “ Tlieories of the Colour of tlie Sky,” ‘ Physical Review,’ XXVI., June, 1908, p. 497. An extensive 
bibliography on sky observations is given in this paper. 
t Ceova, C. R., ‘ Coinptes Rendus,’ CIX., p. 493 ; CXII., p. 1178 ; also ‘ Annales de Chimie et de Physique,’ (6), XX., p. 480. 
X Zettwfch, G-., “ Eicerche sul Bleu del Cielo,” translated in the ‘ Phil. Mag.,’ (6), IV., August, 1902, p. 199. 
§ Majoeaxa, Q., “ On the Relative Luminous Intensities of Sun and Sky,” ‘ Phil. Mag.,’ (6), I., May, 1901, p. 555. 
[[| The writer is indebted to Dr. Otto Klotz, of the Dominion Observatory, Ottawa, for calling his attention to the work of 
Exuee along these lines. Exnee, ‘ Sitzungsbericht d. K. Akad. d. Wissen., Wien, M. N. Klasse,’ 1909, vol. 118, IIa. A 
summary of Exnee’s work and of the observations of Wiesnise and Scheamit, W., is given by Abbot, C. G-., in his recent 
book, ‘ The Sun,’ p. 299 (Appleton and Co., 1911). A problem of somewhat the same natui'e as the qiresent one is considered 
by Gold, E., “ The Isothermal Layer of the Atmosphere and Atmospheric Radiation,” ‘ Roy. Soc. Proc.,’ A, vol. 82, 
1909 .—Note added December 31, 1912.] 
^ Abbot, C. G., “Report on the Astrophysical Observatory,” ‘Annual Report of the Smithsonian Institution,’ 1911, 
p. 65. 
