PHILOSOPHICAL TRANSACTIONS.

I. Transmission of Sunlight through the Earth's Atmosphere.

By Captain W. de W. Abney, C.B., D.C.L., F.R.S.

Received April 7, Read May 5, 1892.

PART II.--Loss of 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 sunlightt 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 Frstc and the Author, as dctailcd in the 13akerian Lecture in 1886. The absorption coeflqcient for the ditbrent thicknesses of atatmosphere at sea level was found by tmeasurements made principally at South Kensington, and these were compared WIth measures taken at the Riffel, above Zermat, at a height of aboutt 8000 feet. I't was shown thatt when the air tthickness is representted by x, the minirotan intensity for each ray of the spectrum can be fairly represented by I't Ie -'�'t't", and the average intensity by I' :---I{ -'mat, I' and I being the transmited and original intensifies, and X 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 tthickness, and a a constant. This corresponds to a -t t corrected for refraction, when 0 is the zenith distance.

This result was arrived at by taking the areas of the curves of luminosity as tbrmed from the actual obtervahons as a measure of the lm.mnost[y of t.he focal white light which was decomposed into a spectrum. This was admissible, for in Ote paper already referred to, it was proved that the sum of luminosities of different rays is equal to their luminosity when compounded.

XXIII.--Objections to the Use of the Fot'mula.

The formula involving X -t is the formula theoretically deduced by Lord .RAynEran for the scattering of light by small pm-tieles, and its adoption in this research is open

MDCCCXCIII. A. B 20.1.93