Captain Abney also showed the satisfactory conditions which existed in using the crater of the positive pole of the electric arc light as a source, and by diagrams illustrated the inferiority of an incandescent light for the purpose, owing to the deficiency of violet and ultra-violet rays. Having thus settled the source of illumination and the kind of apparatus to employ, he next considered the conditions under which the sensitive salts were to be exposed. The action of ordinary sensitizers was explained and demonstrated by experiments, from which point the results of certain colored sensitizers were considered. Thus, various aniline dyes were proved to be bromine absorbents, and likewise, more or less, to be capable of being acted upon by light in those regions of the spectrum they absorbed. The result of the two effects was to produce a developable image of the spectrum just in those parts to which the salt of silver was sensitive, and also in the parts where the dye itself was acted upon. The latter effect was traced to the organic matter being oxidized in the presence of the sensitive silver salt. The sensitizing effect of one silver compound upon another was then gone into, and experiments and photographs showed where two salts of silver were in contact with one another, and without an energetic sensitizer being at hand, that the one when acted upon by light absorbed the halogen liberated from the other through the same cause and that a new molecule was formed. This was of importance, since in photographic spectroscopic researches a conclusion might be arrived at that a body suffered absorption in those regions of the spectrum where this interesting reaction took place, whereas in reality the phenomenon might be due to the silver salts employed. This was another pitfall for the unwary. Again, it became necessary in studying photographic action to make sure that the effect of radiation was only a reducing action, and that the results were not vitiated by some other action.