256 



NA TURE 



\7tUy 30, 1874 



Vou have already seen how exceedingly important it was to 

 use a slit instead of a round hole in ihese experiments. It was the 

 verdict of Wollaston, and it was the verdict of Becquerel and 

 Draper, as I have shown you to-night with regard to photo- 

 graphy. You have also seen thac we can use the circular 

 corona as a slit equally well. 



Now if we take a long slit and divide it into as many 

 portions as we choose, we see at once the improvement 

 that we introduce into spectroscopic photography. All we 

 have to do is to divide that slit into portions, as it were, 

 by letting a window run down the slit, and when the 

 window has arrived at the second part of the slit, let in 

 light from a new source. This principle has been carried out 

 praclically in the following manner : — A rectangular brass plate 

 71 mm. long, and 35 mm. broad, slides in grooves in front of 

 the slit of the spectroscope, and a window 4 mm. high, cut out 

 of this plate, leaves a portion of the slit of this length exposed. 

 A small pin presses firmly against the face of one of the sliding 

 plates, and a row of small shallow holes or notches is drilled in 

 the plate so as to intercept it in its upward or downward move- 

 ment at those points where the pin falls into a notch. The dis- 

 tance between the notches is precisely the same as the height of 

 the opening cut in the sliding plate, so that the movement ot this 

 plate from one notch to another corresponds to a distance equal 

 to the height of the exposed part of the slit, and the spectra 

 compared are confronted, so to speak, absolutely ; the upper 

 edge of one spectrum abuts against the lower edge of the other, 

 and the coincidence, or want of coincidence, between lines in 

 the two spectra can thus be determined with the greatest pre- 

 cision. The spectroscope employed contains three prisms of 

 45° and one of 60° ; its observing telescope is replaced by a 

 camera with a 3-in. lens by Dallmeyer of about 23 in. focal 

 length for the use of which I am indebted to Lord Lindsay. 

 With this arrangement — the spectrum being received upon a 

 sensitised ^ plate — the portion between the wave-lengths, 3,900 

 and 4, 500, can be obtained at once in good focus. A ray of 

 sun'ight, reflected from a heliostat mirror so as to fall upon the 

 slit-plate, is brought to a focus by means of a double convex lens 

 just between the carbon poles of an electric lamp, while 

 a second convex lens placed between the lamp and the 

 collimator tube, serves to cast an image of the sun or 

 of the electric arc upon the slit-pIate. Supposing, now, 

 we wished to compare the iron spectrum with that of the 

 sun : the sun's image in sharp focus on the slit-plate is first 

 allowed to imprint its spectrum on the prepared plate. The 

 ray of sunlight is then cut off, the sliding plate moved up or 

 down till the pin catches in the next notch, and the image cf the 

 arc, passing between an upper pole of carbon and a lower pole 

 consisting of a carbon crucible containing a fragment of iron, is 

 allowed to fall on the portion of tlie slit thus exposed. 



Let me show you some photographs illustrating this de- 

 scription. Here is a single photographic plate on which 

 the new method has enabled us to register no less than four 

 different spectra ; those of you who are familiar %vith photo- 

 graphic processes will immediately see how it is that the num- 

 ber is not forty instead of four. Having a slit of a certain 

 length, if I open all the length of that slit at once I should get a 

 spectrum the breadth of which would depend upon the length 

 of the slit ; but if I commence operations by allowing the light 

 first to come through one small portion of the slit, then we shall 

 get the light from the particular metal which I employ in the 

 electric arc falling on one part of the plate, and registering 

 itself on the photographic plate. Then, if I close up that part uf 

 the slit, and open another one, I shall be able, through that 

 newly opened part of the slit, all the rest being closed, to photo- 

 graph on the plate the spectrum of another substance, say iron. 

 Then, having used up that part of the plate, I can close that 

 portion of the slit, I can bring my window lower down, and there 

 we have the spectrum of cobalt. The window has been brought 

 farther down, and there we have the spectrum of nickel, so that 

 we have, as the work of some eight or nine minutes at the out- 

 s'de, a photograph — not a ]ierfect one in this case, but this was 

 the first one taken on this method — which will register with the 

 most absolute and complete accuracy and certainty not less than 

 1,000 lines. Now a careful student of those lines, working as 

 hard as he can, thinks himselt very fortunate if he can lay down 

 ten an hour. Therefore, as ten an hour are to 1,000 in seven 

 minutes, so is the eye to photography in these matters. 



I have a photograph of a somewhat similar nature, which I am 

 anxious to place belore you. We have here an absolute com- 

 parison rendered possible, by means of photography, between the 



lines of the spectrum of iron and the lines of the spectrum of the 

 sun. Vou see that in the case of most of the thick lines, you get 

 a thick line in the solar spectrum corresponding with the lines of 

 the iron. And, more than this, you see, I hope, all of you, that 

 these lines of iron are of different lengths. The reason of that is 

 that I have been careful to photograph on the plate the lines due 

 to the various strata of iron vapour, from the rarest vapour, 

 which is obtained at the outside of the electric arc, to the densest, 

 which occupies the centre of the core, and you will see the most 

 beautiful gradation as we pass from the outside part of the spec- 

 trum to the inside. This inside part represents the complete 

 spectrum of the core, and the outside the incomplete and almost 

 mono-chromatic spectrum of the vapour which surrounds the 

 denser core in the middle of the spark ; thus we have practically 

 reduced the spectrum of iron to one line, instead of 460. That 

 is the first photograph of the kind which has been taken ; I say 

 that, not because I am proud of it, but because you all know how 

 enormously photographic processes are likely to be developed 

 the moment, not one individual, but a great many, try their hands 

 upon them, so that an enormous improvement upon what you 

 now see may be anticipated. Not only have we developed, in 

 the application of photography to spectroscopy, a valuable ally to 

 Science, as we have in the application of photography to astro- 

 nomy — and you know what that has done, and what it is going 

 to do — but we have, I believe, what we may almost call a new 

 chemistry, some day to be revealed to us by means of photo- 

 graphic records of the behaviour of molecules. Recollect that 

 the difference between the iron spectrum of one line and the iron 

 spectrum ofh^twcen 400 and 500 lines is simply due to the differ- 

 ence in '' .. rangements of the molecules or atoms of iron in 

 ih" »- ', .>; of the electric arc and its exterior. There is one 

 q..i^aon which all lovers of the spectroscope may ask of photo- 

 graphers, and that is this, why should we any longer be confined, 

 in registering spectra, to the more refrangible end of the spec- 

 trum, when one of the very first spectra of the sun that was ever 

 taken was a complete photograph of the spectrum, including not 

 only the blue, the green, and the yellow, but the red, and the 

 extreme red? I think that if photographers will study the action 

 of light on molecules, and read that extraordinary paper of 

 Becquerel's, and will give those who are familiar with the spectro- 

 scope, and those who are anxious to promote the progress of 

 spectroscopic research, a means of extending photographic regis- 

 tration, not only into the green part of the spectrum, which they do 

 already with difficulty, but to the extreme red, then the use of the 

 eye will almost entirely be abolished in these inquiries. And 

 although no one has a higher estimate than myself of the extreme 

 importance of the eye, I think that the more it is replaced by per- 

 manent natural records in these inquiries, the better it will be for 

 the progress of Science. 



J. Norman Lockyer 



SCIENTIFIC SERIALS 



The current number of the Quarterly yonrnal of IMicroscopk 

 Science contains several papers of interest. Dr. Michael Foster 

 commences with an article On the terra Endothelium, in which 

 he proves that the word is etymologically pure nonsense, 

 Ruysch's word epithelium signifying that it covers papilkis. His 

 endothelium must be understood to mean tJiat it is inside a papilla^ 

 It is also valueless for other reasons : for if it is defined as that 

 epithelium developed from the germinal mesoblast, the epithelium 

 of the Wolffian ducts, of the ureters, and of MuUer's ducts would 

 have to be included. Therefore the term is insignificant and 

 must be abolished. Monoderic and polyderie are proposed as 

 terms to indicate that tlie cells form one or several layers. — The 

 second part of Prof, llaeckel's interesting Gastrsea theory fol- 

 lows, in English. In it the systematic and the phylogenetic sig- 

 nification of the Gastr.Tja theory and the ontogenetic succession 

 of the system of organs are discussed, as well as the bearing of 

 the whole on the theory of types. The author is so prolific in 

 his introduction of new words, the definitions of many of which 

 are to be found in other publications, that a llaeckel Glossary in 

 the next number of the Journal would not be out of place, to 

 assist readers in the full appreciation of that illustrious biologist's 

 very suggestive theory. — Air. J. W. Groves explains his method 

 of arranging and cataloguing microscopic specimens. — A paper 

 follows by Mr. K. C. Baber On picro-carminate of ammonia as 

 a microscopic staining fluid, in which he explains M. Ranvier's 

 method. The great advantage of this reagent is shown to con- 

 sist in its staining tissues in a series of colours varying from red 



