By investigating the conditions under which it was possible to prepare a photographic paper of uniform and constant sensitiveness, and ascertaining the means by which the darkening of the paper on insolation could be accurately compared with a standard tint, it was found comparatively easy to construct an instrument capable of measuring the chemical action of light effected at any point on the earth’s surface by the total sunlight and diffuse daylight under the most widely varying circumstances of climate and atmospheric condition.
This joint research, begun in 1855, occupied its authors until 1862. Roscoe did the greater part of the experimental work, and after his election to the professorship in Owens College in 1857 he spent his long vacations in Heidelberg in continuing the inquiry. The results were communicated in a series of memoirs to the Royal Society and are published in the Philosophical Transactions.[5]
Subsequently he pursued the subject alone or in conjunction with others. In a short paper published in 1863 he gave the results of a series of measurements of the chemical brightness of various portions of the solar disc made by means of standard photographic paper according to the method described by Bunsen and himself in their last communication;[6] and in 1864 he described a method of meteorological registration of the chemical action of total daylight based on a modification of that originally used by Bunsen and himself. The account of this method was made the Bakerian Lecture in 1865, and is published in the Philosophical Transactions of that year.[7]
In this paper he gives the results of consecutive observations on each day for nearly a month at about midsummer, and compares the chemical action of light at Manchester at the winter and summer solstices, and the vernal and autumnal equinoxes. The wide variation in the chemical action of light at different periods of the year was illustrated by the fact that if the integral of that on the shortest day be taken as the unit, that upon the equinox will be represented by 7, and that upon the longest day by 25.
In 1866 he and Mr. Baxendell contributed a joint note to the Royal Society on the relative chemical intensities of direct sunlight and diffuse daylight at different altitudes of the sun. They showed from observations made at Manchester and at Heidelberg that the ratio of the chemical intensity of direct to diffuse sunlight for a given altitude at different localities is not constant, but varies with the transparency of the atmosphere, and that this ratio does not in the least correspond with the value of visible intensity as estimated by the eye, the action of the atmosphere being 17·4 times greater upon the chemical than on the luminous rays when the sun’s altitude is about 25° and 26·4 times greater when it is 12°.[8]
With a view to the introduction of the instrument into meteorology, and as part of the routine work of an observatory, he caused a regular series of measurements to be made during two years at the Kew Observatory, under the direction of Dr. Balfour Stewart; and in order to gain further knowledge of the variation in the chemical action of light in different areas of the earth’s surface, he sent the writer in 1866 to Pará, on the river Amazon, 1° 28´ south of the Equator. The selection of this particular place arose from the circumstance that his cousin, now the Right Hon. Charles Booth, was proceeding to the Brazils in connection with the establishment of a new line of steamers, and it was arranged that the writer should accompany him, in order that he might make photometric observations en route, going and returning, and at the same time make a series of determinations of the amount of carbon dioxide in sea air, night and day, with a view of testing the truth of an allegation by a French chemist, that it was subject to a diurnal variation, depending upon the intensity of sunlight.
The special form of apparatus arranged for making photometric observations at sea proved to be ill-adapted to the purpose. But even if its performance had been good, the conditions under which it had to be employed were incapable of furnishing valid results. Accordingly the writer elected to remain at Pará in order to obtain the required observations at that place, until such time as he could return by the succeeding steamer. He was thus enabled to make a much more extensive set of observations than was originally contemplated, and under conditions which ensured trustworthy measurements.
The following letter, dated May 12, 1866, and received at Pará, refers to these matters. The allusion to Agassiz arose from the circumstance that the great naturalist was at the time engaged in work on the Amazons and its tributaries under the auspices of the Emperor Dom Pedro.
Roscoe himself was busily engaged with his vanadium researches.