His first research was undoubtedly suggested by Bunsen. As already stated, it resulted in the classical investigation on the laws regulating photochemical action. It was already known that a mixture of equal volumes of hydrogen and chlorine on exposure to light lost its characteristic colour, and was converted into hydrochloric acid, readily soluble in water; and Bunsen conceived the idea of making this reaction the basis of a method of measuring the relative amount and activity of those light-vibrations which are mainly concerned in effecting chemical change. As a matter of fact the idea was not new, for, unknown to Bunsen, it had already been adopted by Draper, of New York, who had, as he states in his paper in the _Philosophical Magazine_ for December 1843: invented an instrument [based upon the same reaction] for measuring the chemical force of the tithonic rays, which are found at a maximum in the indigo space, and which from that point gradually fade away to each end of the spectrum. It perhaps says little for the assiduity with which young Roscoe read the original chemical literature of his time that he should only have knowledge of Draper’s remarkable papers some thirteen years after they were published in the English journals. But it is only due to him to say the chemical students of University College in those far-off days had fewer opportunities of access to original literature than they now enjoy. Be this as it may, Roscoe’s discomfiture at being thus anticipated was of no long duration. Do not (wrote Bunsen) let your discovery of Draper’s work disconcert you.… It appears to me that the value of an investigation is not to be measured by whether something is described in it for the first time, but rather by what means and methods a fact is proved beyond doubt or cavil, and in this respect I think that Draper has left plenty for us to do.