Discoveries and Inventions of the Nineteenth Century

atmosphere by its gases and aqueous vapour. This demonstration of the existence of iron and nickel in the sun is an interesting pendent to the known composition of many meteorites which reach us from interplanetary space. Kirchhoff was led to believe that the central part of the sun is formed of an incandescent solid or liquid, giving out rays of all refrangibility, just as white-hot carbon does; that round this there is an immense atmosphere, in which sodium, iron, aluminium, &c., exist in the state of gas, where they have the power of absorbing certain rays; that the solar atmosphere extends far beyond the sun, and forms the corona; and that the dark sunspots, which astronomers have supposed to be cavities, are a kind of cloud, floating in the vaporous atmosphere. During total eclipses of the sun, certain red-coloured prominences have been noticed projecting from the sun’s limb, and visible only when the glare of its disc is entirely intercepted by the moon. Fig. 228 represents a total eclipse, and will give a rude notion of the appearance of the red prominences seen against the fainter light of the corona, which extends to a considerable distance beyond the sun’s disc. Now, two distinguished men of science simultaneously and independently made the discovery of a mode of seeing these red prominences, even when the sun was unobscured. M. Janssen was observing a total eclipse of the sun in India, and the examination by the spectroscope of the light emitted from the red prominences showed him that they were due to immense columns of incandescent hydrogen, for 439he recognised the red line and blue lines which belong to the spectrum of this gas (see No. 12, Plate XVII.). Mr. Norman Lockyer at the same time also succeeded in viewing the solar prominences in London without an eclipse. He found a red line perfectly coinciding in position with Fraunhofer’s C line and that of hydrogen, another nearly coinciding with F, and a third yellow line near D. Soon after this, Dr. Huggins discovered a mode of observing the shape of the red prominences at any time, by using a powerful train of prisms and a wide slit, so that the changes in the forms of the red flames can be followed. Now, since the red prominences give off only a few rays of particular refrangibility, it is not difficult to understand that the light of the sun might be, as it were, so diluted by stretching out the spectrum, by means of a train of many prisms, that almost only the red rays, C, should enter the telescope, and occupy the field with sufficient intensity to overpower all others, and produce an image of the object from which they originated. The nature of this action may be illustrated thus: If we hold vertically a prism, and look through it at a candle-flame, we may perceive a lengthened-out image of the flame, showing the succession of prismatic colours, and formed, as it were, of a red image of the flame close to a yellow one, and so on, but presenting no defined form. If, still viewing this spectrum, we introduce into the flame on a platinum wire a piece of common salt, we shall perceive a well-defined yellow image of the candle start out, because the rays which are emitted by the incandescent sodium, being all of one refrangibility, the prism simply refracts without dispersing them. The dispersion which weakens the light of the continuous spectrum by lengthening it out, does not sensibly detract from the brilliancy of the bright lines, as their breadth is scarcely increased—they are refracted but not dispersed. Hence, when a sufficient number of prisms is employed, the bright lines of the solar chromosphere may be seen in full sunshine, in spite of the greater intensity of the light emanating from the photosphere, which produces the continuous spectrum. The bright C line is, of course, a virtual image of the slit produced by rays of that particular refrangibility; but by using a very high dispersive power, the slit may be opened so wide that the C rays form in the telescope a red image of the prominence from which they issue, since their light will predominate over that of any rays belonging to the continuous spectrum. Fig. 228.—Solar Eclipse, 1869. In the hands of Mr. Norman Lockyer the science of the physical and chemical constitution of the sun has made rapid progress, and new facts are continually being observed, which serve to furnish more and more definite views. Mr. Lockyer considers that, extending to a great distance around the sun is an atmosphere of comparatively cooler hydrogen, or perhaps of some still lighter substance which is unknown to us. It is this which forms what is termed the corona, or circle of light which is seen surrounding the 440sun in a total eclipse. Immersed in this, and extending to a much smaller distance from the nucleus of the sun, is another envelope, termed the chromosphere, consisting of incandescent hydrogen and some glowing vapours of magnesium and calcium. The brightest part of this envelope, which lies nearest the sun, is that which gives off the red rays by which the prominences may be observed without an eclipse. These prominences have been shown to be tremendous outbursts of glowing hydrogen, belched up with sometimes an enormous velocity from below, since they have been observed to spring up 90,000 miles in a few minutes. Beneath the chromosphere, and nearer to the body of the sun, are enormous quantities of the vapours of the different elements—sodium, iron, &c.—to which the dark lines of the solar spectrum are due. This stratum Mr. Lockyer calls the reversing layer, because it reverses (turns to dark) the lines which would otherwise have appeared bright, just as Kirchhoff’s sodium vapour did in the experiment described on page 437. Beneath the reversing layer is the photosphere, from which emanates the light that is absorbed in part by the reversing layer, and which there is good reason to believe is either intensely heated solid or liquid matter. In 1861 Dr. Huggins devoted himself, with an ardour which has since known no remission, to the extension of prismatic analysis to the other heavenly bodies. The difficulties of the investigations were great. There was first the small quantity of light which a star sends to the spectator; this was obviated by the use of a telescope of large aperture, which admitted and brought to a focus many more rays from the star, and therefore the brightness of the image was proportionately increased. Not so the size of the image: the case of the fixed stars for this always remains a mere point. It was, of course, necessary to drive the telescope by clockwork, so that the light of the star might be stationary on the field of the spectroscope. As the spectrum of the image of the star formed by the object-glass would be a mere line, without sufficient breadth for an observation of the dark or light lines by which it might be crossed, it is necessary to spread out the image so that the whole of the light may be drawn out into a very narrow line, having a length no greater than will produce a spectrum broad enough for the eye to distinguish the lines in it. This is accomplished by means of a cylindrical lens placed in the focus of the object-glass, and immediately in front of the slit. Covering one-half of the slit is a right-angled prism by which the light to be compared with that of the star is reflected into the slit. The light is usually that produced by taking electric sparks between wires of the metal in the manner already described. The dispersive power of the spectroscope was furnished by two prisms of very dense glass, and the spectrum was viewed through a telescope of short focal length. Dr. Huggins’s observations lead him to the conclusion that the planets Mars, Jupiter, and Saturn possess atmospheres, as does also the beautiful ring by which Saturn is surrounded; for he noticed in the spectrum of each different dark lines not belonging to the solar spectrum. Fig. 229.—The Planet Saturn. Passing to the results obtained in the case of the fixed stars, we may remind the reader of the enormous distance of the bodies which are submitted to the new method of analysis. Sir John Herschel gives the following illustration of the remoteness of Sirius—supposed to be one of the nearest of the fixed stars: Take a globe, 2 ft. in diameter, to represent the sun, and at a distance of 215 ft. place a pea, to give the proportionate size and distance of the earth. If you wish to represent the distance of Sirius 441on the same scale, you must suppose something placed forty thousand miles away from the little models of sun and earth. But not only do we know with certainty some of the substances contained in Sirius, but the star spectroscope has taught us a great deal about orbs so remote, that their distance is absolutely unmeasurable. About Aldebaran we know that there are hydrogen gas and vapours of magnesium, iron, calcium, sodium, and some four or five other elements. Generally the lines indicate the presence of hydrogen in these distant suns; but there is, at least, one remarkable exception in α Orionis, the spectrum of which yields no trace of the hydrogen lines, although it is evident that magnesium, sodium, calcium, &c., are present. The spectra of celestial bodies are of several kinds. Many of the stars have, like our sun, a continuous spectrum crossed by dark lines. Such is that of Sirius, No. 10, Plate XVII. Others have, however, both dark and bright lines, and some are marked by only three bright spaces. Of the spectra of the nebulæ some have three bright lines (see No. 11, Plate XVII.), and the bodies producing them are, therefore, to be considered as masses of incandescent gas, while some give continuous spectra. One of the bright lines in the spectra of the nebulæ coincides with one of the hydrogen lines, and another—the brightest of the three—with one of the brightest nitrogen lines; but the third does not agree with any with which it has as yet been compared. The inference from these appearances is that the nebulæ contain hydrogen and nitrogen, but the absence of the other lines of these substances has not been fully explained; although the observation of Dr. Huggins, that when the light of incandescent nitrogen and hydrogen is gradually obscured by interposing layers of neutral tinted glass, the lines corresponding with those in the nebular 442spectra are the last to disappear, seems to suggest a probable solution of the difficulty. Fig. 230.—Solar Prominences, No. 1. There is another very interesting line of spectroscopic research in the power the prism gives us of estimating the velocity with which the distances of the stars from our system are increasing or diminishing. On closely examining the hydrogen lines of Sirius, and comparing them with the bright lines of hydrogen rendered incandescent by electric discharges in a Geissler tube, the spectrum of which his instrument enabled him to place side by side with that of the star, Mr. Huggins was surprised to find that the lines in the latter did not exactly coincide in position with those of the former, but appeared slightly nearer the red end of the spectrum. This indicated a longer wave-length, or increased period of vibration, according to the theory of light, which would be accounted for by a receding motion between Sirius and the earth, just as the crest of successive waves of the sea would overtake a boat going in the same direction at longer intervals of time than those at which they would pass a fixed point, while, if the boat were meeting the waves, these intervals would, on the other hand, be shorter. Hence if the position of the lines in the spectrum depends on the