PHILOSOPHICAL TRANSACTIONS. 
I. Transmission of Simlifit through the Earth's Atmosphere. 
By Captain W. de W. Abney, C.B., D.C.L., F.R.S. 
Eeceived April 7,—Read May 5, 1892. 
Part II. —Loss or Light at different Altitudes. 
Introductory. 
XXII.—In a previous communication (‘Phil. Trans.,’ vol. 178, 1887, A., pp. 251-283) 
the results of the absorption of sunlight by different thicknesses of atmosphere near 
sea level were found from measurements of the luminosity of the entire spectrum. The 
method used was that devised by General Festing and the Author, as detailed in the 
Bakerian Lecture in 1886. The absorption coefficient for the different thicknesses of 
atmosphere at sea level was found by measurements made principally at South 
Kensington, and these were compared with measures taken at the Eiffel, above 
Zermatt, at a height of about 8000 feet. It was shown that when the air thickness 
is represented by x, the minimum intensity for each ray of the spectrum can be fairly 
represented by I' = jg--oi3.rA-*^ average intensity by I' = I' and I 
being the transmitted and original intensities, and \ the wave length. 
Further, it was shown that even if the absorption of each ray was very different 
from the above, the integral absorption was very accurately expressed by a~~, z being 
the air thickness, and a a constant. This corresponds to corrected for refraction, 
when 9 is the zenith distance. 
This result was arrived at by taking the areas of the curves of luminosity as formed 
from the actual observations as a measure of the luminosity of the total white light 
which was decomposed into a spectrum. This was admissible, for in the paper already 
referred to, it was proved that the sum of luminosities of different rays is ecpial to 
their luminosity when compounded. 
XXIII. —Ohjections to the Use of the Formula. 
The formula involving is the formula theoretically deduced by Lord Eayleigh 
for the scattering of light by small particles, and its adoption in this researcli is open 
MDCCCXCIII.—A. B 
