Discoveries and Inventions of the Nineteenth Century

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 periods of vibration, that position will be shifted towards the red end when the luminous body is receding from the earth with a velocity comparable to that of light, and towards the violet end when the motion is one of approach. The change in refrangibility observed by Mr. Huggins corresponded with a receding velocity of 41·4 miles per second, and when from this was subtracted the known speed with which the earth’s motion round the sun was carrying us from the star at the time, the remainder expressed a motion of recession 443amounting to about twenty miles a second, which motion, there is reason to believe, is chiefly due to a proper movement of Sirius. These deductions from prismatic observations are of the highest value astronomically, since they will eventually enable the real motions of the stars to be determined, for ordinary observation could only show us that component of the motion which is at right angles to the visual ray, while this gives the component along the visual ray. In the same manner, it is inferred that Arcturus, a bright star in the constellation Boötes, is approaching us with a velocity of fifty-five miles per second. Fig. 231.—Solar Prominences, No. 2. When the solar spots are examined with the spectroscope, the dark image of the slit produced by the hydrogen line, F, is observed to show a strange crookedness when it is formed by rays from different parts of the spot. This distortion is due to the same cause as the displacement of the stellar lines, namely, motions of approach or recession of the masses of glowing hydrogen. Mr. Norman Lockyer, to whom we are indebted for the most elaborate investigations of the solar surface, has calculated, from the position of the lines, the velocities with which masses of heated hydrogen are seen bursting upwards, and those which belong to the down-rushes of cooler gas. Velocities as great as 100 miles per second were, in this way, inferred to occur in some of the storms which agitate the solar surface. Two drawings of a solar storm, given by Mr. Lockyer, are shown in Figs. 230 and 231. These are representations of one of the so-called red prominences, the first giving its appearance at five minutes past eleven on the morning of March 14th, 1869, and the last showing the same ten minutes afterwards. 444The enormous velocity which these rapid changes imply will be understood when it is stated that this prominence was 27,000 miles high. “This will give you some idea,” says Mr. Lockyer, “of the indications which the spectroscope reveals to us, of the enormous forces at work in the sun, merely as representing the stars, for everything we have to say about the sun the prism tells us—and it was the first to tell us—we must assume to be said about the stars. I have little doubt that, as time rolls on, the spectroscope will become, in fact, almost the pocket companion of every one amongst us; and it is utterly impossible to foresee what depths of space will not in time be gauged and completely investigated by this new method of research.” The light of comets has also been examined by the spectroscope, and many interesting results arrived at. Our limits do not, however, permit us to enter into a discussion of these interesting subjects. Fig. 232 is a section of another of Mr. Browning’s popular instruments, which is named by him the “Amateur’s Star Spectroscope.” It exhibits very distinctly the different spectra of the various stars, nebulæ, comets, &c. Fig. 232.—Section of Amateur’s Star Spectroscope. The reader who is desirous of learning more of this fascinating subject is referred to Dr. Roscoe’s elegant volume, entitled, “Lectures on Spectrum Analysis.” This work, which is embellished with handsome engravings and illustrated by coloured maps and spectra, gives a clear and full account of every department of the subject, and in the form of appendices, abstracts of the more important original papers are supplied, while a complete list is given of all the memoirs and publications relating to the spectroscope which have been published. This brief account of the spectroscope and its revelations, which is all that our space permits us to give, will not fail to awaken new thoughts in the mind of a reader who has obtained even a glimpse of the nature of the subject, especially in relation to that branch of which we have last treated, for in every age and in every region the stars have attracted the gaze and excited the imagination of men. The belief in their influence over human affairs was profound, universal, and enduring; for it survived the dawn of rising science, being among the last shades of the long night of superstition which melted away in the morning of true knowledge. Even Francis Bacon, the father of the inductive philosophy, and old Sir Thomas Browne, the exposer of “Vulgar Errors,” believed in the influences of the stars; for while recognizing the impostures practised by its professors, they still regarded astrology as a science not altogether vain. It was reserved for the mighty genius of Newton to prove that in very truth there are invisible ties connecting our earth with those remote and brilliant bodies—ties more potent than ever astrology divined; for he showed that even the most distant orb is bound to its companions and to our planet by the same power 445that draws the projected stone to the ground. And now the spectroscope is revealing other lines of connection, and showing that not gravitation alone is the sympathetic bond which unites our globe to the celestial orbs, but that there exists the closer tie of a common constitution, for they are all made of the same matter, obeying the same physical and chemical laws which belong to it on the earth. We learn that hydrogen, and magnesium, and iron, and other familiar substances, exist in these inconceivably distant suns, and there exhibit the identical properties which characterize them here. We confirm, by the spectroscope, the fact partially revealed by other lines of research, that the stars which appear so fixed, are, in reality, careering through space, each with its proper motion. We learn also that the stars are the theatres of vast chemical and physical changes and transformations, the rapidity and extent of which we can hardly conceive. There is, for example, the case of that wonderful star in the constellation of the Crown, which, in 1866, suddenly blazed out, from a scarcely discernible telescopic star, to become one of the most conspicuous in the heavens, and the bright lines its beams produced in the spectroscope revealed the fact that this abrupt splendour was due to masses—who can imagine how vast?—of incandescent hydrogen. This brightness soon waned, and τ Coronæ Borealis reverted once more to all but telescopic invisibility. The seeming fixity of the stars is an illusion of the same nature as that which prevents