The Absorption of Light by the Atmosphere 1

Abstract

THE great attention that has been paid during the last few years to the subject of photometry has brought into prominence the problem of the amount of light absorbed by the atmosphere. At the same time, the improvement that has taken place in the instrumental means, which renders possible the detection of minute changes in lustre, has required the use of accurate corrections by which the effect of the earth's atmosphere can be eliminated from the observations. The corrections which have been applied to photometric measures have been based generally on empirical or interpolation methods rather than on a strictly physical basis. There are several reasons which have contributed to this unsatisfactory condition of the problem. The difficulty of computing the length of the path of the ray of light in its passage through our atmosphere, the want of homogeneity in the constitution of the atmosphere itself, our ignorance of the law of the temperature gradient at considerable heights above the surface, and of the distribution of water and dust particles near the surface, have all complicated a subject the theory of which under ideal limiting conditions may not be very difficult. Bouguer left a very satisfactory theory, based, however, on the assumption that the path of the ray was rectilinear. Laplace attacked the subject from the side of the theory of refraction, but practically did not much advance it. From that time onward, the question has rather been left in the hands of observers, who have been content to make their observations homogeneous by the employment of an interpolation formula, based on the results of their actual practice.