Sept. 12, 187a] 



NATURE 



531 



were examined. Briefly it may be stated he found that if 

 a spectrum was allowed to fall on a sensitised daguerro- 

 type plate, which had received a preliminary exposure 

 to white light, a remarkable phenomenon took place—a 

 phenomenon which was also observable if weak white 

 light were allowed to fall on the plate during its exposure 

 to the solar spectrum. In developing such an image with 

 mercury vapour, the blue, or most refrangible end of the 

 spectrum was impressed in the usual way, that is to say, 

 the Fraunhofer lines showed as dark lines on a lighter 

 back-ground ; at the red or least refrangible end of the 

 spectrum, however, the Fraunhofer lines were seen as 

 light on a darker back-ground ; in other words, the photo- 

 graphic action was reversed, the neutral point of no 

 action lying somewhere in the yellow. On studying a 

 picture taken by this means, it was observed that in all 

 cases the darkest Fraunhofer lines had the same tint, and 

 that effect of light lines upon dark background, or dark 

 lines upon the light background, were caused by alteration 

 in tint of the background itself. Could this effect have 

 anything to do with the oxidation ? if it had it would 

 indicate that the rays in the least refrangible end of the 

 spectrum must accelerate oxidation; for it must be re- 

 membered that the plates had received an exposure to 

 white light, either before, or during, exposure to the spec- 

 trum, and that the red rays prevented the development of 

 the effect of the white light. 



Now Draper had failed to get the same result on films 

 of collodion containing the sensitive silver compounds, 

 though he had obtained the reversal of the least refran- 

 gible end of the spectrum on such plates. If the theory 



H e FED c n <t A. 



,1. . .■• ■ , ■ '■„ ' ,- . 



Fig. 3. 



Fig. 4. 



of oxidation held good for daguerrotype plates it ought 

 also to hold good for the collodion films, and experiment 

 decided once again in favour of the theory. Collodion 

 films, which held in situ the blue form of silver bromide 

 already alluded to, and which had been proved to be 

 sensitive to the red end of the spectrum in the ordinary 

 sense of the word (i.e., that a proper negative picture of 

 it could be obtained as it could of the blue end), were 

 the subjects of experiment. It is evident, if the red rays 

 were accelerators of oxidation, that in order to get a 

 positive picture of the red end {i.e., one answering to the 

 reversal of the Fraunhofer lines in Draper's daguerro- 

 type plates) the films should be exposed to the spectrum 

 whilst in some oxidising medium, weak enough by itself, 

 not totally to obliterate during the time of exposure 

 to the spectrum any preliminary exposure which should 

 be given to them, and yet strong enough to do so 

 and to destroy the reducing action of the red rays, if 

 tTiese latter possessed a power of accelerating oxidation. 

 Ozone, hydroxyl, nitric acid, and other oxidising agents, 

 completely corroborated the idea that all the red rays 

 had the power of accelerating oxidation, as the positive 

 pictures of the red end here obtained, and in some case 

 of the blue end, with negative pictures of the yellow and 

 ultra-violet. The strength of the oxidising solutions was 

 very small ; thus, when nitric acid was used, four drops 

 to a couple of ounces of water was found sufficient to 

 c.iuse this remarkable action to take place, whilst if the 

 nliric acid were reduced in quantity, or omitted altogether, 



the effect of the ordinary negative picture was obtained 

 in that part of the spectrum (Figs. 3 and 4). On the 

 other hand, when the strength was increased, the image 

 disappeared altogether. Figs. 3 and 4 show the results 

 indicated above ; the shaded portions show where the 

 spectrum was photographed in the usual way, the white 

 portions indicate where the reversed action took place. 



We are not sure but we believe that Draper used silver 

 iodide as his sensitive salt in the experiments with col- 

 lodion, in which he failed to obtain these phenomena. 

 The iodide is insensitive to the red end of the spectrum 

 under ordinary conditions of exposure, being usually ex- 

 posed in the presence of a solution of silver nitrate which 

 clings to it after taking it out of the bath. It was found, 

 however, if this free silver nitrate were washed away and 

 if the exposure to the spectrum took place in the oxidising 

 medium, after a preliminary exposure to white light, that 

 this reversing action, as it is called, of the red end of the 

 spectrum was obtained ; and under certain conditions if 

 the silver nitrate were removed, that the same results 

 could be obtained even when the plate was not exposed 

 in this oxidising medium. 



Now regarding the reversal in the blue indicated above 

 how can it be accounted for ? By the very same theory, 

 only an abandonment of the hypothesis that the least 

 refrangible end of the spectrum a/one is an accelerator 

 of oxidation becomes necessary. In all comparative ex- 

 periments made with the daguerreotype plate and the 

 collodion film the difference of these conditions must be 

 remembered. In the former the halogen hberated by the 

 action of the light on the iodide combines immediately 

 with the metallic plate forming fresh sensitive compound ; 

 in the latter the thickness of the sensitive compound has 

 a limit, and much of it is altogether inoperative, the outside 

 of the particles alone being available for the reducing 

 action of light, and the halogen has to escape or be ab- 

 sorbed as best it may. In a collodion film it is manifest 

 that the reduction of all the sensitive compound avail- 

 able must take place after a time, and when this is the 

 case, if the same rays which effect reduction likewise 

 accelerate oxidation, that the latter effect of the rays will 

 have unimpeded action. So much for the theory, does 

 experiment prove or disprove it ? It is evident if the 

 hypothesis be correct, that a film which is exposed to the 

 action of light in a medium free from oxygen, or in one 

 which is an absorbent of oxygen, should be incapable, 

 on development, of showing this reversal of the ordinary 

 action of light. 



The results again showed that the theory was borne 

 out, for it was found impossible to obtain a reversal of the 

 image when so exposed. Here, then, we have a probable 

 explanation of the phenomenon known as solarisation, to 

 which allusion has already been made ; it seems to be an 

 oxidation of the undeveloped image. 



W. DE WiVELESLlE ABNEY 



(To be continued^ 



NOTES 



Dr. O. Finsch has resigned his appointment as Gustos of the 

 Museum of Bremen, and, as soon as the publication of his work 

 on the results of the German Siberian Expedition of 1877 is 

 completed, will leave Europe oh a scientific mission to the 

 Pacific and Australia. Dr. Ludv^•ig, late Assistant in the Zoo* 

 logical Museum of Gottingen, has been appointed Dr. Finsch's 

 successor. 



We notice the death at Cuenca, in Ecuador, on June 20, of 

 Gustav Wallis, the botanist. He was born at Limeberg, in 

 Detmold, May i, 1830. In i860 he was commissioned by the 

 Lindens, the great horticulturists of Brussels, to gather new 

 varieties of plants in South America, and during eight years, 

 alncst without ce?sation, he traversed Brazil, Peru, Ecuador, 



