RECENT RESEARCHES IN PHOTOGRAPHY. 719 



by M. Becquerel with every plate tried and witli collodions contain- 

 ing different quantities of chlorophyll. 



It must be admitted, then, that a film exerting selective absorp- 

 tion in intimate contact with a sensitive film of silver bromide or 

 iodide affects the latter in those parts of the spectrum where the 

 selective action is taking place. Here, surely, is a wide field for inves- 

 tigation, and one the importance of w^hich will be at once obvious to 

 the physicist. Practically also, when the precise conditions of action 

 are made known, valuable results may be anticipated from the appli- 

 cation of this principle to science and to art. Since the year 1842, 

 when M. Becquerel photographed the whole solar spectrum from the 

 extreme violet to the extreme red, and when Dr. J. W. Draper photo- 

 graphed the violet, blue, and extreme red, no successful attempts have 

 been made to imprint the least refrangible end of the spectrum ; and 

 this, when we consider the great importance that the study of the solar 

 spectrum has assumed of late years, and the painful or even dangerous 

 character of prolonged eye-observation, is to us a matter. of wonder. 

 M. Becquerel's result, it will be remembered, was obtained by a film 

 of silver iodide, first insolated or exj)osed to diffused light and then to 

 the action of the spectrum. Here, again, is another question — the pre- 

 cise action of insolation on sensitive plates — demanding explanation 

 at the hands of the physicist. The practical aspect of Dr. Vogel's 

 discovery need not here be discussed at length. Attention may be 

 called to the well-known difficulty of getting reds or yellows to im- 

 print themselves in portraiture, a difficulty which now bids fair to be 

 overcome. 



Then, again, in what we must consider as a higher sphere of prac- 

 tical utility, great advantage to the study of solar physics is likely to 

 accrue. In point of fact, the photographic method of comparing spec- 

 tra described in a recent communication to the Royal Society now 

 becomes available. for the whole extent of the solar spectrum, and our 

 knowledge of the true composition of the sun will be thus in course 

 of time recorded permanently on "that retina which never forgets." 



Great results have already been achieved by photography, and 

 greater may be looked for. It must not be forgotten that in this 

 most interesting branch of chemical physics we are in a period either 

 of provisional hypothesis, or, worse still, of no hypothesis at all, so 

 that valuable additions to our knowledge of physical and chemical 

 laws should be forthcoming. The changes wrought by a beam of 

 light on sensitive surfaces are sometimes physical and sometimes 

 chemical. We may appropriately recall here the fact that mechanical 

 pressure upon a sensitized surface of a silver salt acts in the same 

 manner as a ray of light, giving a dark stain under the action of re- 

 ducing agents. The experiment of Grove also, in which an electric 

 current is set up by the incidence of a beam of liglit upon a prepared 

 Daguerreotype plate, should not be forgotten. The equivalence be- 



