tube A, was spread out more and more through each prism as it passed, and fell in a spectrum on the object glass, c, of the telescope B, through which they examined it. They soon found that in order to mark the exact position of the bright lines of each gas upon the spectrum, they wanted some fixed measure, and it occurred to them that the black lines of the solar spectrum, which never change, would make a good scale with which to compare all the others. So they arranged their spectroscope in such a manner that one-half of the slit was lighted by the sun and the other half by the flame of a gas. In this way No. 2, Plate I., would appear above, and No. 3, for example, immediately below it. While doing this they could not help remarking that the bright yellow line of the sodium spectrum, No. 3, was exactly in the same position as the black line, D, in the solar spectrum; and Kirchhoff found that when he passed a faint ray of sunlight through a flame of sodium (so as to make These lines are really double when seen in a powerful spectroscope, but they appear single in a small instrument. CH. XXXIII. THE SOLAR SPECTRUM. 325 the two spectra, 2 and 3, cover each other), the yellow line exactly filled up the black line with its light. He now wished to see how bright he could make the solar spectrum. without overpowering the light of the sodium, so he let the full sunshine pass through the sodium flame. To his great astonishment he saw the black line at D start out more strongly than ever. The sodium flame had revenged itself for being overpowered by absorbing some of the yellow light of the sun! This suggested to him the idea that the black line D must be caused by the white light from the sun passing through sodium vapour before it reaches us. There was a very simple way of proving whether this were so; for burning solids, you remember, give a continuous spectrum (1, Plate I.), therefore, if he could produce a dark line by passing the light of a burning solid through sodium vapour, he would imitate one of the defects in sunlight. So he burned a lime-light, and when he had the continuous coloured band in his spectroscope, he burned a sodium flame between the limelight and the prism. The experiment was quite successful; the dark space, D, started out upon the spectrum, and thus he proved beyond doubt that burning sodium vapour absorbs in white light exactly those rays which it gives out itself when burning. He repeated the experiment with other burning metals, such as potassium and strontium, and always with the same result. Each burning gas absorbed in the white light exactly those rays which it gave out itself when burning. The black lines on the solar spectrum were now explained, for each one of them must imply that some particular ray of sunlight has been absorbed by a gas between the sun and us, and it must have been absorbed near the sun, as Fraun hofer had pointed out, because the lines are different in light which comes from the stars, showing that in that case it has passed through other kinds of gases. Therefore Kirchhoff concluded that round the solid or liquid body of the sun, which gives out white light, and would of itself produce a continuous spectrum, there must be an atmosphere of gases of different kinds, which absorb or destroy particular rays of light, and prevent them reaching us. If this is the case, it is clear that we can tell from the lines in the spectrum what gases and vapours there are in this solar atmosphere. For example, there must be sodium which cuts off the rays which ought to come to D, and there must be also iron, magnesium, calcium, chromium, potassium, rubidium, nickel, barium, lead, copper, zinc, strontium, cadmium, cobalt, uranium, cerium, vanadium, palladium, aluminium, titanium, and hydrogen, for the bright lines of all these metals are replaced by dark lines in the solar spectrum, showing that the white light from the body of the sun must have passed through their gases. Dr. Huggins and Dr. Miller examine the Stars by Spectrum Analysis, 1862.-Only a few months after Kirchhoff had proved that the black lines in the solar spectrum reveal to us what elements exist as gases around the sun, two English chemists, Dr. Miller, who died a few years ago, and Dr. Huggins, who is still living, began to try the same experiments with the other heavenly bodies, and the study has since been carried still farther by Mr. Lockyer. Their instruments were now much more perfect than those which Fraunhofer had used, and they were able to see the effects of our own atmosphere upon sunlight, for when the sun is setting and its light has to pass through a long layer of air before it reaches us, faint lines appear on the CH. XXXIII. GASEOUS NEBULÆ. 327 spectrum, because some light is absorbed by the vapours in our atmosphere. Now, when Miller and Huggins examined. the light which comes from Jupiter, they found three or four lines like those caused by our atmosphere, showing that Jupiter must have an atmosphere partly, but not entirely like ours. Mars and Saturn also both showed these atmospheric lines, and so did Saturn's rings, proving that a similar atmosphere must spread over them also. But our moon gave none of them, and this agreed with other evidence in showing that the moon has no atmosphere. They next passed on to examine the light of the stars, and this was by no means an easy task, because the stars are so far off that their light is very faint and difficult to catch. Nevertheless they proved that round one star, called Aldebaran (No. 5, Plate I.), there must be an atmosphere of hydrogen, sodium, magnesium, calcium, iron, tellurium, antimony, bismuth, and mercury, and you will notice that the last four of these are not found in the sun. In the light of the star Betelgeux, in the constellation Orion, and in another star, called ẞ Pegasi, no hydrogen is found, but it is found in all the other stars, together with many other substances. some of the stars there are besides, lines which are not found by the burning gas of any known substances on our earth. In Dr. Huggins proves that some Nebula are Gaseous, 1864. And now we come to a very interesting experiment. You will remember that astronomers doubted Sir W. Herschel when he suggested (p. 275) that some of the nebulæ are not made of tiny stars, but of gas which is forming into stars. In 1864 Dr. Huggins began to examine these nebula with the spectroscope, and he found that they did not give a band of colour with dark lines upon it as the stars do, but a few faint lines on a dark ground, exactly as gases do which we burn here on earth. If you compare the spectrum of sodium (No. 3), or of hydrogen (No. 4), with the nebula spectrum (No.6), you will see at once that the nebula spectrum is made by a gas, and so the truth of Sir W. Herschel's idea was proved, and there can be now no doubt that some of the nebulæ are composed of gaseous matter; chiefly, so far as we can learn, of nitrogen and hydrogen. Mr. Alexander Herschel examines the Spectrum of Falling Stars. I have said that it was difficult to examine the spectrum of the stars and nebulæ, but something which to an ordinary observer seems still more wonderful has been lately done. Mr. Alexander Herschel has actually caught the light of falling stars in the spectroscope, and in this way has discovered that some of them give a continuous spectrum, showing that they are solid bodies, while others give a gas spectrum, on which are the bright lines of potassium, sulphur, and phosphorus, but chiefly of sodium. Such wonderful facts as these about far-distant suns and sun-matter, we have learnt, and are still learning by means of spectrum analysis. The whole study was only begun. fifty years ago, and it is in the works of living men that you must look for the details of its history. But though many eminent names are connected with it, those of Fraunhofer and Kirchhoff should always be remembered as the chief founders of the science. ་ Chief Works consulted.-Roscoe's 'Spectrum Analysis;' 'Edinburgh Review,' vol. cxvi. ; 'Philosophical Magazine,' 1860; Proctor, 'The Sun;' Tyndall's Lectures on Light;' 'Half-hours with Modern Scientists;' Kirchhoff's Researches on the Solar Spectrum,' 1862; 'Encyclopædia Britannica,' art. 'Optics;' Ganot's Physics;' Wollaston, On Dispersion '-'Phil. Trans.' 1802; Lockyer, The Spectroscope.' ་ |