April 2g, 1875] 



NA TURE 



505 



possibly have been made, it will be forwarded for criticism 

 to the Committee (on Geometrical Teaching) of the 

 British Association and to other mathematical authorities. 

 The object, we further learn, is, if possible, to get the 

 sanction of the British Association ; and this backing the 

 opinion of the large number of mathematical teachers who 

 now form the Association, will, it is hoped, lead the 

 examining bodies of the country to act with perfect im- 

 partiality in considering the merits of those pupils who 

 have been trained in accordance with the methods of the 

 Syllabus as contrasted with the favourers of Euclid. 



From the Report we gather that the principal work of 

 the Association is e.\pected to be completed in another 

 two years ; it is not attempted to forecast what will be 

 its subsequent work. Perhaps, as has, we believe, been 

 suggested, it may become an Association for the Improve- 

 ment of Mathematical Teaching. 



As the publications of the Association are for private 

 circulation, we cannot go into further detail ; we may, 

 however, say that it has done good work in having been 

 the moving cause of five valuable Presidential Addresses. 



LETTERS TO THE EDITOR 



[ The Editor does not hold himself respojisible for opinions expressed 

 by his eorrespondenis. Neither can he undertake to return^ 

 or to correspond with the writers of^ rejected manuscripts. 

 No notice is taken oj anonymous co7Hinunications.'\ 



Influence of Pigments on the Photographic Image of 

 the Spectrum 



When, some time since. Prof H. Vogel announced the dis- 

 covery that the addition of a pigment to a film of bromide 

 of silver made it sensitive to light of the colour which that 

 jiij^ment gave it, though it had not been so previously, many — 

 ii.c\ td I might say most — photographic chemists doubted the 

 acciiiacy of his observations and the existence of any such law. 

 His experiments were rehearsed by most of them, and the 

 reports were, in almost every case, contradictory of his conclu- 

 sions. There were powerful iz priori reasons for doubting, 

 amongst which the chief was, in my own opinion, that if a film 

 coloured (say) red were sensitive to red light, it could not be 

 developed under red light, but would fog, and would therefore 

 be unworkable, which was not found to be the case. Another 

 was, that the use of tinted films, well known for a long lime, had 

 only resulted in an universal retardation of all colours. It 

 was, moreover, contrary to the known analogies of actinism that 

 a purely mechanical admixture irrespective of any chemical 

 quahty should produce changes of so purely chemical a natuie 

 as those which are the basis of photographic action. 



By the kindness of Mr. Lockyer I was enabled to experiment 

 at his laboratory at South Kensington with the same plates 

 (Col. Wortley's tinted tilms) that Prof Vogel had based his 

 discovery on, and, as I expected, found tl^e results quite other 

 than those the professor had announced. Although a protracted 

 exposure ( seventeen minutes) was given, and the moie lefran- 

 gible lines were quite buried by halation, no line was shown 

 which did not appear in the oidirary wet cx-llcdion film. 



That careful and excellent photographic chemist, Mr. Spiller, 

 President ol the London Photographic Society, Dr. Van 

 iVIonckhoven, Mr. Caiey Lea, and numerous others, amongst 

 whom I am enabled, by his personal assurance, to name Dr. 

 J. W. Draper, unquestionably the first living authority on spec- 

 trum photography, as well as his not less well-known son. Prof. 

 Draper, have also followed Vogel in his experiments without 

 obtaining any confirmation of his law. 



Up to this time the only testimony confirmatory of his views 

 offered is that of Becquerel, who, as the most marked instance 

 of success, gives this— that chlorophyl (a green substance) 

 gives great sensitiveness to red rays ! That most indefatigable 

 and precise experimentalist, Mr. Carey Lea (of Philadelphia, 

 U. S. A.), in the course of a long series of experiments, unfor- 

 tunately interrupted by his ill-health, showed that while coral- 

 line in a film did add shghtly to tlie length of the spectrum 

 image, other red pigments produced no effect whatever, and that 

 saHcine, which has no colour, produced more effect than coral- 

 line. Etit if chlorophyl, a green substance, is sensitive to red 



light, anihne green, so far as my own experiments go, produces 

 no effect whatever except prolongation of the exposure necessary. 



Now, without m the least disputing the prolongation of the 

 spectrum photograph as claimed by Prof Vogel, or depreciatin- 

 the importance of his results, it seems to me that we are in a 

 position to assume that he is entirely mistaken in the nature of 

 the law he deduces, and that these results are due to purely 

 chemical causes, in no wise dependent on colour, though in a 

 few cases the colour may coincide with the chemical cause in 

 such a way as to afford apparent confirmation of his hypothens 



It must be remembered that Dr. Draper has long ago shown 

 that all the rays have chemical activity, and that he has, without 

 any such aid as Vogel has called in, produced complete photo- 

 graphic spectra ; and has also shown ihat different substances 

 decompose under different rays. Becquerel's experience with 

 chlorophyl gives a clue to the connection between these disco- 

 veries and Vogel's results, if collated with a series of phe- 

 nomena resumed by Dr. Draper (fiom observations by Dr. 

 Gardner) in the interesting papers by him on the " Distribution 

 of Chemical Force in the Speclrum " * :— " In Dr. Gardner's 

 paper there are also some interesting facts respecting the 

 bleaching or decolorisation of chlorophyl by light. He used an 

 ethereal solution of that substance : — ' The first action of light is 

 perceived in the mean red rays, and it attains a maximum in- 

 comparably greater at that point than elsewhere. The next 

 part affected is the indigo, and accompanying it there is an 

 action from + 10-5 to + 36 o of the same scale (Herschel's), 

 beginning abruptly in Fraunhofer's blue. So striking is this 

 whole result, that some of my earlier spectra contained a per- 

 fectly neutral space hom - 5 o to -h 20-5, in which the chloro- 

 phyl was in no way changed, whilst the solar r.icture in the red 

 was sharp and of a dazzling white. The maximum in the indigo 

 was also bleached, producing a linear spectrum as follows : — 



in which the orange, yellow, and green rays are neutral. Tliese, 

 it will be remembered, are active in forming chloiophjl.' . 

 I have quoted these results in detail, because they illustrate in a 

 striking manner the law that vegetable colours are datroyed by 

 rays comphineiitary to those that have produced them, and furnish 

 proof that rays of every refrangibility may be chemically active." 

 (P. 7, "Researches in Actinic Chemistry.") 



Dr. Draper goes on in this memoir to est.iblish a second pro- 

 poiition to this effect: "That the ray effective in producing 

 chemical or molecular changes in any special substance is deter- 

 mined by the absorptive property of that substance." This 

 proposition, laid down in 1S41, seems to me to contain the 

 explanation of all the phenomena of chemical or molecular 

 change in photographic films ; and if I might be permitted to 

 offer an hypothtsis supplementary to the proposition, serving, if 

 dciiwtistrahle, as corollary to it, it would be that if two sub- 

 stances having diflerent absorptive properties are simultaneously 

 (or nearly so) subjected to the action of white light, in molecu- 

 lar contact the change in one of them may be communicated 

 to the other mechauiially. Thus, bromide of silver, which is 

 not sensitive to the red ray, being placed in contact with chloro- 

 phyl, which is sensitive to that colour, the action of the red ray 

 is communicated from the latter to the foimer substance, pro- 

 ducing what may be designated as a sympathetic molecular effect. 

 But in Older that this may obtain, it is necessary that the 

 auxilisry substance applied to influence the sensitive photo- 

 graphic film should be in it--elf sensitive to other rays than 

 those which decompose the silver bromide. This would account 

 for the effect of chlorophyl and peihaps (or the original experi- 

 ment which attracted tlie attention of Prof Vogel, as the dry 

 plates of Col. Wortley with which it was made contain salicine 

 in their preservative as well as an aniline red in their substance, 

 and Mr. Carey Lea has shown that salicine has the effect which 

 Vogel claims for the colour. 



If this is tenable, it follows that the object of our researches 

 should be to discover those substances which have an indepen- 

 dent susceptibility to actinic action, but for different rays than 

 those which form the basis of the film experimented on. The 

 results so far obtained in this direction, even those of Vogel him- 

 self, are, it seems to me, quite as capable of explanation by the 

 hypothesis I have offered as by that of an arbitrary effect of 

 colour; in confirmation of which we have only experiments (thus 

 far made public) by Prof Vogel himself. 



It seems to me incredible that, if such a law existed, such 

 * " Researches in Actinic Chemistrj', Memoir Second," &c. John William 

 Draper, M.D., LL.D., New York. 



