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]