ynne ii, 1874] 



NA TURE 



tog 



ON SPECTRUM PHOTOGRAPHY* 



'T'lIOSE of you who know best how the Society of Arts 

 ■^ always places itself in the forefront of any movement 

 which is likely to benefit mankind by the application of the [ 

 various sciences to the practical affairs of life, may recollect 

 that, as nearly as may be thirty years ago, the dawn of a ; 

 new science was brought before an audience in this room. If I j 

 look, no longer to the J ournal, but to the ' ' Transactions," of the 1 

 Society of Arts, Manufactures, and Commerce, as far back as j 

 the year 1S43, + I find a paper there by the late Mr. Claudet, who 

 then gave an account of the progress which had been made up to 

 that time in an art and a science which is now pefectly familiar 

 to all of you ; I refer to photography. It is exceedingly 

 curious that his lecture on the origin of this science, and my 

 present lecture on the application ol photography to spectrum 

 analysis are complementary to each otlier, so much so that one 

 may almost say that Mr. Claudet's lecture, admirable though it 

 was, was incomplete, because he did not show in it, as of course 

 he could not, how certain matters which he referred to in that 

 lecture have been dealt with in the light of moJern science. 



If yo'.i carry yourselves back to the year 1839, some four years 

 before the lecture to which I refer was delivered, you will recol- 

 lect Mr. Niepce had at that time brought pho!ography to a more 

 ]iractical realisation than it had been by any of his predecessors. 

 He had then for some years allied himself with Daguerre, and 

 the daguerrotype was already in existence. The action of iodine 

 on silver, first discovered by Fox Talbot, had been fixed by the 

 vapour of mercury, i Now, in the daguerrotype we had not the 

 action of light in its ordinary sense ; and men's minds were 

 very much exercised as to what could be the real cause 

 of the effects which were then being revealed. Mr. Claudet, 

 in his lecture, points this out in a most admirable way, 

 and I will summarise, if you will allow me, some of the principal 

 points to which he alludes. You had a beam of light falling on 

 a plate. On this plate was a certain chemical compound. What 

 part of the sunlight, or w.as it sunlight at all, which so acted upon 

 this compound, that you got an image more or less permanent ? 

 What more natural than that this question should be investigated 

 by means of various tinted glasses? The solar beam which the 

 experimenters then used they made to pass through glass, now of 

 one colour, and now of another. I can show you, by means of 

 this electric lamp, nearly what they did. Imagine the lamp to 

 be the sun ; in the path of the beam differently coloured glasses 

 are placed. We have now the action of a red giass ; we now 

 change the red glass for another one, and now we have the 

 action of a green glass. There was an immense deal of differ- 

 ence of opinion concerning the action of light as investigated in 

 this way. In fact, I shall have shortly to show that Mr. Claudet 

 and a very distinguished French physici.st, M. Becquerel, wete 

 considerably at variance with regard to one particular point 

 which ramc out from this kind of investigation. But we had mt 



long to wait. Sir J. Ilerschel, in the year 1839, pointed out that 

 it was not a question of investigating these new qualities of light 

 at all by means of coloured glasses ; they should be investi- 

 gated b,' means of the spectrum. In three papers, com- 

 municated to the Royal Society in the years 1839, 1S40, and 

 1S42, he showed that the only philosophic way of investigating this 

 problem was really by obtaining a pure spectrum, such a one as 

 I now throw upon the screen. Vou see that we have, at once, 

 in different parts of this spectrum, exactly what we get at different 

 times when we deal with red glass, yellow glass, orange glass, 

 green glass, blue glass, and si> on. And having such a spectrum 

 as this to deal with, and supposing such a spectrum thrown on to 

 the photographic pLite, itiscptite clear to all of you that if there 

 were something magical or unknown in the red rays which gave 

 us this new action on the molecules of the particular chemical 

 compound employed, or whether this magic really resided in the 

 blue rays, that we should at once have this pointed out to us in 

 the most unmistakeable manner, by action in the part of the 

 plate on which the red rays fell, or in the part of the plate on 

 which the blue rays fell. 



Now, although .Sir John Herschel was the first, in this country, 

 to point out the extreme importance of this point of view, he 

 was by no means the only one. Then, as now, there were dis- 

 tinguished Americans who were well to the front, and among 

 them was Dr. Draper, the father of another Dr. Draper whom I 

 shall have to speak of by and by. Those of you who are 

 familiar with the enormous step in advance which was taken in 

 spectroscopic investigations by WoUaston, who substituted a slit 

 for a round hole, will perhaps be somewhat surprised to find that 

 the first observations were conducted by throwing a converging 

 beam o*^ sunlight, giving an achromatic image of the sun, on the 

 plate, through a prism. This method of procedure of course 

 did not go .so far as a better one might have gone, but it went a 

 considerable way. Sir J. Herschel, from his observations m»de 

 in this manner, stated that he had found a new kind of light — a 

 new prismatic colour, "lavender grey," altogether beyond the 

 blue end of the spectram, such as you have seen it on the screen 

 — altogether bevond the hlnc end of the spectrum, not the nd 

 end. Prof. Draper, on his part, also came in the miin to the 

 same conclusion, stating that he had discovered a "latent 

 light." 



When we have come from the year 1S39 to the years 1S42 

 and 1S43, we find a great advance — an advance, just the same 

 as far as photography goe5, as WoUaston' s advance on Newton 

 was with regard to spectroscopic observation. Both Becquerel 

 and Draper introduced, instead of this achromatic image of the 

 sun, the simple arrangement of throwing sunlight through a 

 slit and a proper combination of lenses on to a plate. The 

 result was that on June 13, 1S42, Becqucr.4 did whit 

 I may venture to call a stupendous feat." He did what 

 has never been done since, so far as I know. lie photographed 

 the. whole solar spectrum wi h nearly all the lines registered by 



Ji\k\ 



-Reduced copy of i;ecquerers pliotoQr.Tph lA tlic t omplcte soL^r spectrum take: 



the hand and eye of Fraunhofer. I do not mean merely the blue 

 end of ihe spectrum, as you may imagine, but the complete spec- 

 trum, from the *' latent light " — the ultra-violet rays of Draper— to 

 the extreme red end. Draper also did something like the same thing, 

 but not quite tlie same thing, in what he calls a ** tithonographic 

 representation " of the solar spectrum. He gives certain lines in 

 the extreme visible blue part of the spectrum, § certain other 

 lines, which none but Becquerel had ever seen before (Draper's 

 work being done nearly a year later), and in the ex'reme red — 

 beyond the visible red of the spectrum — he gives other lines which 

 even Becquerel had not photographed. Thisofcoursewassuchatre- 

 niendous revelation to both these men that as you can imagine a con- 

 siderable discussion arose. Becquerel lound, from an absolute com- 

 parison between the Fraunhofer lines which he had photographed 



• A Canlur Lrcture delivered at llie .Society of ArU, Nov 24, 1871, by 

 J. Norm:in Lockyer. F.K S. 



t Vol. Iv. p it). 



J Fox Talbot, rhilomplikal Magazin,-, \a\ xxii. p, 97. 



5 rldlosophicil M.is.iziu.; vol. xxd p. 360, 18 »3. tor his earliest vi'ork 

 see Journal of ttie Franklin Institute for the year 1837. 



and the Fraunhofer lines which Fraunhofer himself had registered, 

 evidence in favour of the fact that this new chemical agent which 

 was astonishing the world, whatever it was, vv.as not something 

 absolutely and completely independent ot the visible rays. 

 Draper, on the other hand, in his "tithonographic representa- 

 tion," had, for ^ome photographic reason or other, not succeeded 

 in registering the lines in the yellow, orange, and green part of 

 the spectrum, although he had fixed the lines in the blue, in the 

 extreme violet, and in the extreme red ; and he considered him- 

 self justified by his experiments in coming to exactly the opposite 

 conclusion to that at which Becquerel had arrived, namely, thit 

 the light, whatever kind ot light it might be, which wis at work 

 in effecting this chemical change which rendered photography 

 possible, was something abiolutely and completely iadepend;iit 

 of the ordinary light which the retina receives. 



This was in the year 1S43. I need not tell you that by the 

 year '845, in which year Mr. Claudet read another paper before 

 this Society, further investigations by means of the spectrum had 

 • " B.bliotheqiie universelle de Geneve." t xx.slx.-xl . t8(J. p. 3«t ] 



