III. Transmission of sunlight through the Earth’s atmosphere

Abstract

The observations were made by means of the colour photometer which General Festing and himself introduced last year, and which they described in the Bakerian Lecture for 1886. They extended over more than a year, the object being to ascertain the intensity of the different rays in the solar spectrum after passing through various thicknesses of the atmosphere. Owing to the unpromising results obtained by Langley with his bolometer experiments, it was not anticipated that the variation in the intensities of the different rays would obey any law, but subsequent investigation showed that as a rule the intensity of any ray obeyed the law enunciated by Lord Rayleigh, in that I' = I ε - kxλ -4 , where I and I' are the initial and transmitted wave-lengths, x the thickness of the medium through which the ray passed, and k a constant, λ being the wave-length. The standard illuminating value of the spectrum was taken from observations made in Switzerland at 8000 feet altitude on September 15 at noon. The other observations were made at South Kensington. It was found that with the wind in the proper quarter the sky at the latter place was as pure in colour as in the country, and that measures made on the days on which there was apparently no haze gave results which when combined together gave a minimum value for k of 0·0013. A mean of the results showed that k = 0·0017.