282 



NA TURE 



\Auo. 13, 1874 



conclusion one of the plates was washed in alcohol and 

 water in order to remove the yellow colouring matter with 

 which the plate was coated, and it was then found to 

 have lost, in accordance with Dr. Voxel's anticipations, 

 its sensitiveness for the green rays. The pcculi.ir action 

 of the Wortley dry plates was thus shown to be due to the 

 coating of colouring matter, and the next step made by 

 Vogel was to seek some substance which especially ab- 

 sorbed in the yellow, and at the same time .icted as a 

 sensitiser by fixing the free bromine liljeratcd by the 

 action of light upon the silver bromide. Both these ends 

 are fulfdled by the coal-tar colour known as coralline. A 

 plate dyed with this substance and exposed to the spectrum 

 exhibited two maxima of photographic action, one the ordi- 

 nary maximum in the indigo (near G), and the other, almost 

 as strong, in the yellow, thus affording complete confirmation 

 of Dr. Vogcl's views. Aniline green* was next tried. This 

 dye is stated to aljsorb the red rays, and a corresponding 

 increase of sensitiveness for the red rays was observed, 

 the photograph again presenting two maxima of activity, 

 the one in indigo and one in the red coinciding in position 

 with the .absorption band of the dye. Thus Dr. Vogel's 

 results may be summarised by saying that a dyed film of 

 silver bromide exhibits maxima of sensitiveness in those 

 regions where the colouring matter exerts its maximum 

 of absorptive power, but the precise conditions under 

 which these results can be obtained must be considered 

 at present as unknown, since many observers in repeating 

 the experiments, among others Dr. Van IVIonckhoven,t 

 have failed to obtain other than negative results. 



In a communication made to the French Academy on 

 the 27lh of last month, however, the well-known physicist, 

 M. Edm. Hecquerel, stated that some experiments made 

 at his instigation by M. Deshaies at the Conservatoire des 

 Arts et Metiers had been productive of positive effects, 

 and that some of Dr. Vogel's results with coralline and 

 aniline green had been reproduced. M. Becquerel, how- 

 ever, does not confine himself to bromide films ; similar 

 results have been obtained by iodised collodion in which 

 coralline was dissolved. A most remarkable action was 

 observed also in the case of chlorophyll when this sub- 

 st.ance was used as a tinctorial agent. Although the col- 

 lodion possessed only a faint green colour from the dis- 

 solved chlorophyll, the spectral image was ofamuch greater 

 length than when plain collodion was used. Under tliesc 

 last circumstances the spectrum extended from the uhr.a- 

 violet to between G and F, with the usual maximum of 

 action near G, while with chlorophyll the region of 

 strongest action extended from the ultra-violet to the line 

 E in the green, and at the same time a weaker but yet 

 distinct impression extended from I", to beyond B in 

 the red, with a strong band between C and D. By a 

 close examination of the spectral image a second band 

 of less intensity could be detected on the least refrangible 

 side of the band between C and D, and other still we.\ker 

 bands appeared in llie green. The most striking confir- 

 mation of X'ogel's results is to be found in the fact, ob- 

 served by I\I. Becquerel, that the band between C and D 

 corrcspoiuis in position ivitli the charaeteristie bond of the 

 absorption spectrum of chlorophyll dissolved in collodion. 



* The green referred to is prol).il)ly th.-»t kiiowti as " aUtehyde green." 

 The so-cailcd "iodine green," as 1 have frciiuently observed, irttiisttiits w 

 band in the red 



\ Plutcgrafhic youriial. No. as, June 20, 1S74. 



The same results were obtained by M. Becquerel with 

 every plate tried and with collodions containing different 

 ([uantities of chlorophyll. 



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

 absorption 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 investigation, and one the 

 importance of which will be at once obvious to the phy- 

 sicist. Practically .also, when the precise conditions of 

 action are made known, valuable results may be antici- 

 pated from the application 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 

 photographed the violet, blue, and extreme red, no suc- 

 cessful attempts have been made to imprint the least 

 refrangible end of the spectrum ; and this, when we con- 

 sider 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 exposed to diffused light and 

 then to the action of the spectrum. Here again is 

 another question — the precise action of insolation on sen- 

 sitive 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 imprint themselves in portraiture, a difficulty 

 which now bids fair to be overcome. 



Then .again, in what we must consider as a higher 

 sphere of practical utility, great advantage to the study of 

 solar physics is likely to accrue. In point of fact the 

 photogr.aphic method of comparing spectra described in 

 a recent communication to the Royal Society now be- 

 comes available for the whole extent of the solar spec- 

 trum, 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 photo- 

 graphy, 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 addi- 

 tions 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 some- 

 times chemical. We may appropriately recall here the 

 fact that mech.anical pressure upon a sensitised surface 

 of a silver s.alt acts in the same manner as a ray of light, 

 giving a dark stain under the action of reducing agents. 

 The experiment of Grove also, in which an electric current 

 is set up by the incidence of a beam of light upon a pre- 

 pared Daguerreotype plate, should not be forgotten. The 

 equivalence between light and the other form of force has 

 not yet been est.ablished, and it may not be going too far 

 to conjecture that thermodynamics m.ay possibly in the 

 future have to appeal to the action of light upon a photo- 

 graphic plate. In the meantime we look forward to the 

 promised continuation of Dr. Vogel's researches with no 

 little hope. R. Mei.dol.\ 



