very same lines, the combination of the two sources of light produced darkness.

Kirchhoff at once seized on the interpretation of his discovery; and no nobler generalization has ever rewarded the labours of the physicist. Every vapour, he asserted, has the power of absorbing light of that particular kind which it is capable of emitting; so that precisely as the existence of certain bright lines in a spectrum indicates that such and such glowing vapours are the source of light, so the existence of corresponding dark lines across a rainbow-tinted spectrum, indicates that those same vapours absorb light proceeding from an incandescent

source.

The interest and importance of this generalization are well illustrated by considering the meaning of the very observation which led to it. The existence of a double dark line in the solar spectrum, corresponding exactly in position with the double bright line of glowing sodium, proved that the element sodium-the basis of our familiar salt and soda-exists in the sun which shines upon us from beyond ninety millions of miles!

But Kirchhoff did not stop here. He saw at once that he had a means of analyzing the solar constitution; he proceeded to compare the bright lines in the spectra of different elements with the dark solar lines; and he found that a number of the so-called terrestrial elements exist in the atmosphere of the sun.

The spectrum of iron consists of an almost countless number of bright lines. When Kirchhoff compared these with the dark lines in the solar spectrum, he found that every bright line has its dark counterpart. No doubt can exist, therefore, that iron exists in the sun.

In a similar manner Kirchhoff proved that magnesium, barium, zinc, calcium, nickel, copper, and chromium, exist in the sun. Gold is probably present, though the characteristic lines are not all nor distinctly visible. The lines belonging to silver, mercury, lead, tin, and other metals, have not yet been recognized in the solar spectrum. We are not, however, to conclude that these metals do not exist in the substance of the sun. The negative evidence of the spectroscope, in such a case, is not decisive, as its positive teachings are. The probability, indeed, seems to be in favour of the existence of all those substances in the sun, which we have been in the habit of terming the terrestrial elements; the very fact that we have proved some to be present, leading to the inference that in general respects the sun is constituted as the earth is. The various elements may exist in different proportions in the sun, as undoubtedly they exist in a different condition; but the same materials doubtless constitute both orbs.

And here I may note, in passing, an interesting general conclusion which may be deduced not unreasonably from this result. We cannot suppose that the tiny globe on which we live is the only member of the planetary scheme which is constructed of the same materials as the great centre of the system. In Mercury and Mars and Venus, and passing

beyond the mysterious zone of Asteroids, in giant Jupiter, ringed Saturn, and the distant Uranus and Neptune, there must exist the same general resemblance to the globe which rules over all these orbs. And thus a new argument is afforded for the belief that those other worlds, like our own earth, are intended to be the abodes not only of life, but of creatures possessing intelligence, and so able to apply the various elements to the useful purposes they subserve to ourselves.

We have seen how Kirchhoff at once applied the new mode of research to the examination of the solar structure. But it is obvious that there are no limits to the application of spectroscopic analysis. As certainly as the chemist can avail himself of its teachings in the quiet of his laboratory and in the analysis of substances which are, so to speak, in his hands, the astronomer can analyze by its means the stars which shine from beyond distances so vast that the sun's enormous distance sinks by comparison into utter insignificance.

There were certain practical difficulties in the application of the spectroscope to the analysis of stellar light, into which I need not here enter. Rutherford of America, Fr. Secchi of Rome, and our Astronomer Royal, were not successful in their attempts to obtain trustworthy results. But at length the problem was mastered by the persevering efforts of Professor Miller and Mr. Huggins, who together examined the spectra of a large number of the fixed stars.

Sirius, the brightest fixed star in the heavens, was found to exhibit a spectrum of singular complexity and interest. As this star does not rise to any great height above the horizon in our latitudes, its spectrum could not be examined very satisfactorily. Its light, passing through the denser atmospheric strata, was unsteady and flickering, and the delicate work of comparing the dark lines in its spectrum with the bright lines of various metallic and other spectra was thus rendered exceedingly difficult. Yet the two physicists were able to establish the existence of sodium, iron, magnesium, and hydrogen, in this distant sun.

Aldebaran, the bright star in the head of the Bull, was subjected to a most careful examination. The spectrum of this star is crossed by a multitude of dark lines, the places of more than eighty of which were carefully measured by the two observers. They were able to place beyond a doubt the existence of sodium, iron, magnesium, calcium, mercury, bismuth, tellurium, antimony, and hydrogen, in this red orb, while other elements as yet undetermined obviously exist in its substance. The bright star Betelgeux, in the shoulder of Orion, was subjected to equally careful analysis. The spectrum of this star is remarkable on account of the absence of hydrogen, or at least of the two well-marked lines corresponding to this gas. It may be that the absence of these lines is simply due to the fact that the hydrogen existing in the vaporous envelope of this star gives out as much light as it absorbs or more. In the case of our own sun, for instance, the dark lines of hydrogen are occasionally found reversed, bright lines taking their place, when the VOL 9.

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spectrum of particular parts of the sun's surface are examined. It may be that a similar state of things prevails ordinarily in the case of Betelgeux and those other stars in whose spectra the hydrogen lines are not seen. Professor Miller and Mr. Huggins found that sodium, iron, calcium, magnesium, thallium, and bismuth, exist in the substance of Betelgeux.

About fifty stars were examined, with more or less attentive scrutiny. In all cases the spectra were crossed by a multitude of dark lines, and in nearly all the existence of several well-known terrestrial elements was placed beyond a doubt.

And here again we may pause to consider the interesting lesson which is taught us by these researches. We have seen that the constitution of our sun resembles, in general respects, that of his attendant planets. We may conclude that the same law holds in the case of the fixed stars, which are, we know, suns like our own. Since then the orbs which circle around Sirius and Betelgeux and Aldebaran doubtless contain the familiar terrestrial elements-magnesium and sodium and iron-what conclusion can we form but that these elements exist out yonder in space for much the same purposes as on our own earth-to subserve, that is, the wants of intelligent as well as non-intelligent beings? It would be rash, indeed, to say that since iron exists in such and such a world dependent on Aldebaran, therefore in that world there must be at this moment creatures capable of applying that metal to arts or sciences or manufactures; because we know that for ages iron remained buried in the depths of earth with none to make use of it. But it seems by no means unreasonable to conclude that the stores of iron in that distant world were intended to be used by intelligent creatures, either in past ages, or now, or at some future epoch.

By a singularly fortunate chance, Mr. Huggins was able to apply the new mode of analysis to a fixed star of an altogether exceptional character. In May, 1867, a new star suddenly blazed out in the constellation called the Northern Crown. I say a new star blazed out, but it would be more correct to say that a telescopic star suddenly acquired extraordinary brilliancy-since the place where the bright star appeared had been occupied by a ninth-magnitude star, and this star still occupies its former place, though its suddenly-acquired splendour has long since passed away.

When Mr. Huggins applied his spectroscope to the examination of this interesting object, he found that the spectrum presented an appearance quite distinct from that of any stellar spectrum he had before examined. There was, as usual, the rainbow-tinted streak crossed by dark lines; but in addition to this spectrum, there was a spectrum of bright lines, superposed, so to speak, upon the continuous spectrum. Two of these bright lines corresponded in position with the hydrogen lines. It was evident, therefore, that there had been a sudden outburst of hydrogen flames around that distant orb.

It has been common to speak of the blazing forth of this star as indicating the occurrence of some terrible catastrophe-as though a sun had suddenly increased in splendour a hundred fold or more, and so the orbs circling around him had either been destroyed or converted into lifeless wastes, as would undoubtedly happen to this earth if our sun were to blaze forth with a correspondingly increased splendour. But I think there is little ground for this belief. It seems to me far more probable, that the star round which this great conflagration took place is in reality much nearer, and therefore much smaller, than has been supposed; that it is not in truth a sun, but belongs to that class of small stars seen in the Milky Way and the Magellanic Clouds-stars which bear about the same relation to such suns as Arcturus and Vega as the Asteroids bear to the planets Jupiter and Saturn.

And now let us consider the results of the new analysis as applied to the examination of the mysterious objects called nebulæ. These objects had long been recognized as being for the most part galaxies of starsof suns that is, like our own which lie at enormous distances beyond the bounds of the sidereal system; and it seemed as though the spectroscopist were now about to push the powers of the new analysis to their utmost, by extending thus greatly the range of distance over which it was to operate.

The first observation made on the nebulæ served to shew, however, that there is a distinctive difference between some at least of these objects and stars or star-groups. In place of a continuous spectrum, Mr. Huggins found that the first nebula he examined gave a spectrum of bright lines. At first he could detect only one line, but afterwards he found there were three. One of these corresponded to the green line of hydrogen; another indicated the existence of nitrogen in the substance of the nebula; while he was unable to identify the third line with a line belonging to any known spectrum.

It followed from this observation that certain nebulæ are gaseous, a view which had been strenuously upheld by Sir William Herschel three quarters of a century ago.

Other nebulæ give spectra resembling those of the fixed stars. These therefore may be clusters of stars, though whether they be in reality external galaxies resembling the sidereal system, is a question open to very grave doubt.

More mysterious in many respects even than the nebulæ, comets have been studied by astronomers with but little success, nay, rather with everincreasing perplexity. Hitherto it has not been possible to apply spectroscopic analysis to their study under favourable conditions, because the comets which have appeared of late years have not been very brilliant. Only one, indeed, has had a tail, or been visible to the naked eye. Still, results have been obtained which are full of interest in themselves, and still more in the promise they afford respecting the fruits of spectroscopic research applied to comets as brilliant as Halley's or Donati's.

In all cases the nucleus of a comet has been found to be gaseous and self-luminous, while the outer part or coma seems to shine by reflecting the solar light. As respects the former point there can be no question, because the spectrum of the nucleus consists of bands of light; but since the spectrum of the coma is continuous, it remains not quite clear whether the light of the coma comes from incandescent particles or is simply reflected solar light. The last comet examined with the spectroscope gave a result which is singularly interesting. Mr. Huggins found that the spectrum of the nucleus shews so close a resemblance to the spectrum of carbon, as to leave no doubt that the substance of the comet contains this peculiar element. It is difficult to understand whence the heat comes by which such a substance as carbon is vaporized in the comet's nucleus; but no doubt can exist that the light of the nucleus really does come from the glowing vapour of carbon. Perhaps electrical discharges, excited in some unknown way by the solar action, may take place between discrete particles of carbon, and so the spectrum of glowing carbon-vapour be exhibited. But in the present state of our knowledge it is idle and perhaps unwise to speculate on questions which will probably soon be solved by observation.

The light of meteors cannot readily be submitted to spectroscopic analysis, because these objects flash so swiftly athwart the heavens. But by an ingenious contrivance, devised by Professor Alexander Herschel and carried into practice by Mr. Browning, F. R. A. S., the optician, it has been found possible to examine meteoric spectra during those nights when these objects appear in showers. It has been found that while the light of many meteors comes from incandescent substances, others are in the state of vapour. Some especially of those which fell during the night of August 10th, 1867, were found by Professor Herschel to exhibit the bright line of sodium so distinctly, that he compared the condition of these objects to the flame of a Bunsen's lamp newly trimmed and largely dosed with a supply of moistened salt.

Recently the spectroscope has been applied successfully to resolve certain questions of solar physics which had long perplexed astronomers; and one such question still remains for solution, which physicists hope to see answered during the total eclipse of December next.

The coloured prominences seen round the sun during a total eclipse are phenomena, into whose nature it had seemed almost hopeless to inquire before the invention of the spectroscope. Seen only at intervals of several years, and then only for a few brief minutes, these objects promised to baffle all research. Few men can hope to witness more than three or four total eclipses in the course of their lives, and few, indeed, have seen so many. Thus no astronomer could hope to devote more than about twenty minutes of his life to the study of these strange protuberances; and who could hope in so short a time to give a satisfactory answer to all the perplexing questions suggested by the aspect of these objects? Accordingly, we find that while some