July 30, 1874] 



NATURE 



255 



used, and, more than this, prisms of glass. You will, therefore, 

 quite understand that the photograph extends only a very little dis- 

 tance beyond the lines II. But America was not satisfied with this, 

 and in the person of Dr. Draper, the son of the Professor Draper 

 whose name is so honourably associated with the commencement 

 of work done in photography thirty years ago, has just now jilioto- 

 graphed the solar spectrum far beyond H. A copy of his 

 photograph is on the wall, but unfortunately I have not a copy 

 which I can throw on the screen. 



I have already referred to the extreme importance of photo- 

 graphy in astronomy, and the point that I wish to urge to-night, 

 after what I have stated regarding all the work which has been 

 done up to the present time, is this — That what photography has 

 been in the past to astronomy — what it will be in the future no 

 one can say — such can photography, and such must photography, 

 be to chemists and physicists. Of course, in the way of photo- 

 graphic application, it is scarcely fair to say that a daily photo- 

 graphic record of the prominences around the sun is a question 

 either of physics or of chemistry. But still the method which 

 enables us, or which, I hope, will enable us shortly, to obtain a 

 daily photograph of every prominence which bursts out — although 

 absolutely invisible to our eyes — on tlie sun, is a method which 

 depends on physical laws, and has nothing to do with astronomy 

 in the ordinary sense. If you will allow me, I will show you 

 now on the screen a photograph of a drawing which was made 

 by an eminent Italian observer in India during the last eclipse. 

 It is a drawing made by Prof. Respighi, of the sun's corona, as 

 seen by the spectroscope ; and I hope in the next eclipse we 

 shall not any longer have merely drawings to refer to, but 

 that we shall have a photograph which can be brought here, 

 and which will let us know exactly how the matter stood. You 

 see there on the screen three rings — a red ring, a green ring, and 

 a blue ring. They are red, green, and blue, because the element 

 in that part of tlie sun's atmosphere — hydrogen — gives us lines in 

 the red, green, and blue; and they are rings because the hydrogen 

 atmo6[)herc extends in the most admirably regular way all round 

 the sun. In fact, we may say that, in observations of this kind, 

 we use the corona instead of the slit, and if that is good for the 

 corona it is perfectly obvious to you it is good for the chromo- 

 sphere — for the brighter regions lying closer to the sun than the 

 corona docs — as we know that it gives a line of intense blue, 

 exactly where photography, as it is generally carried on, has its 

 strongest pinnt iVappui in the spectrum ; and it is quite cfear to 

 you that we ought to be able to get a photograph of this every 

 day, just as easily as we saw it in India during the eclipse. 



We will next consider the application of photography, no 

 longer to the mere solar spectrum, but to the physics of the sun. 

 What is the solar spectrum ? It is the continuous spectrum of 

 the sun, minus certain portions where the light of the continuous 

 spectrum has been absorbed. What have been the absorbers ? 

 The gases and vapours, generally speaking, in an excessively 

 limited zone of the sun's atmosphere, lying close to the bright 

 «un we see ; close, I say, to the photosphere. This zone is called 

 the reversing layer. Then if the solar spectrum is the result of 

 the aljsorption of this reversing layer, what will happen to the 

 solar spectrum if the constitution of the layer changes ? Obviously 

 a change in the solar spectrum. Now, recent researches carried 

 on by means of photography show us that if you take any parti- 

 cular vapour in the reversing layer, which you may call A, for 

 instance, and then assume that the quantity of A in the layer is 

 reduced, the absorption of that particular vapour will be reduced; 

 what then will be the result on the photograph of the solar spec- 

 trum ? Some of the lines will disappear. Suppose that this 

 particular vapour which we call A, instead of being assumed to 

 decrease in quantity, increases in quantity, what will happen to 

 the solar spectrum ? The same researches have told us that as 

 its quantity increases its absorption will increase, and that its 

 increased absorption will be indicated by an increase in the 

 number and in the breadth of the lines absorbed. What, then, 

 will happen to the solar spectrum if any change of this kind is 

 going on ? The photograph of a solar spectrum taken, say, to- 

 day, may be different from the photograph of the same part of 

 the spectrum taken at some distant period. What is the distant 

 period we do not yet know — whether three months, six months, 

 six years, or eleven years ; but, at all events, there is reason to 

 think already that if we had a series of photographs of the 

 solar spectrum, taken year by year, we should see great changes 

 in the spectrum. Allow me to show you a photograph of a 

 very limited portion of the solar question, and I will prove 

 my case ; and let me teU you I could not prove my case if photo- 

 graphy had not been called in, because if the existence of any 



particular metal, or of the increase of any particular metal, de- 

 pends on such a small matter as one line among 10,000, what 

 will happen if a man neglects to observe this change ? People 

 will say, " Oh ! in a research of that kind it is altogether excus- 

 able if he has made a mistake." But if you have a series of 

 phenomena recorded by means of a camera on " a retina which 

 never forgets," as Mr. Delarue has beautifully put it, and if you 

 compare those pictures day by day, and year by year, the thing 

 is put beyond all question when you get one line disappearing, 

 or another line appearing. 



Now we have before us a part of the solar spectrum near the 

 line II, and I wish to call your particular attention to one line. 

 We have admirable drawings of the solar spectrum taken about 

 the year i860. If the draughtsman was recording by means of 

 his eye the lines in the spectrum, he would not be very likely to 

 overlook a line darker than some he inserts, but he might easily 

 overlook finer lines. Now, it is a fact that in the most careful 

 map that we have — a map drawn with a most wonderful honesty 

 and splendid skill — aline is absent in the region indicated, which 

 line is now darker than some that were then drawn, and that line 

 indicates the presence of an additional element in the sun — stron- 

 tium. I do not make this assertion thinking that subsequent 

 facts will show the drawing to be wrong, but because I see 

 reason to believe that what we know already of the sun teaches 

 us that it is one of the most likely things in the world that stron- 

 tium was not present in such great quantity in the reversing layer 

 when the drawing was made ; but, however that may be, I think 

 you will see how important it is that this jihotograph, which I 

 have just thrown on the screen, should be compared with photo- 

 graphs made five, ten, fifteen, a hundred, or two hundred, or as 

 many years as you like ahead, and it is in this possible continuity of 

 observation of the solar spectrum, carried on for centuries, that 

 I do think we have in photography not only a tremendous ally 

 of the spectroscope, but a part of the spectroscope itself. Spec- 

 troscopy, I think, has already arrived at such a point, at all 

 events in connection with the heavenly bodies, that it is [almost 

 useless unless the record is a photographic one. I am glad to 

 say that only to-day I have had a letter from Dr. Draper, who 

 tells me he has at last succeeded in getting an admirable photo- 

 graph of the spectrum of a star. Now that is of the very 

 highest importance, because the sun is nothing but a star, and 

 the stars are nothing but distant suns ; and as long as we 

 merely investigate the sun, however diligently or admirably we 

 do it, and neglect all the others, it is as if a man who might have 

 the whole realm of literature to work at should confine himself to 

 one book, and that book proliably not a good representative of 

 the literature of the country he was examining into. 



So much for the application of photography to what may be 

 called the celestial side of spectroscopy ; but let me tell you that 

 this, so far as spectroscopy is concerned, does not exist. To the 

 spectroscope all nature is one, and it is absolutely impossible to 

 make a single observation, either on a sun, or a star, or a comet, 

 without bringing chemical and physical considerations into play ; 

 and it willjbe a regrettable circumstance if chemists employ the 

 spectroscope in terrestrial chemistry— they have not done much 

 in that way yet— without taking the sun and all the various stars 

 of heaven into counsel, because the spectroscope is absolutely 

 regardless of space, and; shows us that the elements which are 

 most familiar to us here, or at all events a good many of them, 

 are present in the most distant stars, and the spectroscope shows 

 us those elements existing under conditions which are absolutely 

 impossible here. 



There is another point, too — .spectroscopy is, above all 

 things, molecular. We arc dealing with the ultimate atoms, 

 or molecule^;, or whatever you like to call them, when, by 

 means of the spark, we drive a substance into vapour. And 

 if chemists, for instance, will simply ask themselves which 

 substances have their lines reversed in the solar spectrum, 

 I think, before they h.Tve thought that problem out — that 

 very simple problem, as it seems — there will \ be such a 

 Hood of hght thrown upon terrestrial chemistry, that the only 

 wonder will be that it has not been seen before, years and years 

 ago. These, you will say, are theoretical applications. It is 

 perfectly true ; and there are a great many other theoretical appli- 

 cations that it would be my duty, as it would be my pleasure, to 

 bring before you, if time permitted. But that is not all. I have 

 to refer to the application of the spectroscope in what are 

 considered by some people more practical directions. The 

 more you deal with the most abstruse considerations of 

 Science, the more likely you are to get practical applications 

 out of them. 



