430 
MR. LOUIS VESSOT KING ON THE SCATTERING AND 
DiACiRAM XI.— Relative Intensities from Sky and Snn Calculated from Mount IRilson and 
JRashington Ohservations. 
The curves given in Diagram XL represent the ratio zenith skyjsun for equal solid angles, the total 
radiation covering all wave-lengths being taken in both cases. The curve represents the formula 
a,[^^T(0,0<^A/|^"E(0 dX, 
where w is the solid angle subtended by the sun, i.e., w = 27r(l - cos 16') = 6’80 x 10“^. 
Curves 1. refer to Mount Wilson and include the extreme and mean solutions in the expression for 
T(0, (). 
Curve II. refers to Washington and represents the mean solution of the integral equation only. 
Since the ratio just described is that most easily obtained in measurements of sky radiation, the curves 
just given are appropriate for a comparison of theory'with observation. 
Diagram XII. — Relative Intensities from Zenith Sky, Calculated for Different JFave-Lengths from 
IFashington Observations. 
The curves given in Diagram XII. are drawn on the same plan as those described in Diagrams II.-VII. 
for Mount Wilson. In the present instance the numerical values from which the curves are drawn are 
given in Table VIII. Intensities corresponding to the mean solution of the integral equation are alone 
given, since the presence of “ dust ” at low-level stations is an extremely variable factor. This factor gives 
rise to a sharp bend in the attenuation curve for Washington shown in Diagram I. in the neighbourhood 
of 'GlOp. The effect on the intensity of zenith sky is very sharply marked and gives rise to a finite 
discontinuity in the intensity curve corresponding to that wave-length. In reality the bend of the 
attenuation curve in Diagram 1. cannot be as sharp as the intersection of two straight lines, so that the 
discontinuity in the sky intensity curve in this neighbourhood is probably represented by a sharp peak 
(represented in the drawing by the dotted portions of the curves) which arises from the effect of “ dust ” in 
scattering the long-wave radiation. The prominence of this peak in the intensity curve must be an 
extremely variable factor, and may possilily account for discrepancies in observations on sky-radiation; e.g., 
Nicols* mentions as a result of his measurements that sky intensities show far greater relative intensities 
in the longer wave-lengths than one would expect from Rayleigh’s theory. The effect of “dust” is an 
illustration of the effect of several groups of molecular complexes or small particles on radiation travelling 
through a medium containing them; each group which is able to produce a sharp bend in the attenuation- 
curve at any wave-length is also able to produce a corresponding maximum in the intensity-curve of the 
scattered radiation, the whole effect resembling a broad emission band in its spectrum, f 
* NicoiiS, loc. cif., p. .502. 
[t Important experiments describing the transition from lateral scattering to selective reflection are described by 
Wood, R. W., under the name “resonance radiation.” These are described in the ‘Clark University Lectures,’ Clark 
University, 1912, p. 101 .—Kote added December 31, 1912.] 
