Societies and Academies

Abstract

LONDON Royal Society, March 31.—“On the relation between sun's altitude and the chemical intensity of total daylight in a cloudless sky.” By Henry E. Roscoe, F.R.S., and T. E. Thorpe, Ph.D. In this communication the authors give the results of a series of determinations of the chemical intensity of total daylight made in the autumn of 1867 on the flat tableland on the southern side of the Tagus, about 81/2 miles to the south-east of Lisbon, under a cloudless sky, with the object of ascertaining the relation existing between the solar altitude and the chemical intensity. The method of measurement adopted was that described in a previous communication to the Society,† founded upon the exact estimation of the tint which standard sensitive paper assumes when exposed for a given time to the action of daylight. The experiments were made as follows:—I. The chemical action of total daylight was observed in the ordinary manner. 2. The chemical action of the diffused daylight was then observed by throwing on to the exposed paper the shadow of a small blackened brass ball, placed at such a distance that its apparent diameter, seen from the position of the paper, was slightly larger than that of the sun's disk. 3. Observation No. I was repeated. 4. Observation No. 2 was repeated.