almost out of view, it flickered up again for a while, but soon after it died out, so as to be entirely invisible. Whether a powerful telescope would still have shown it is uncertain, but it seems extremely probable. It may be, indeed, that this new star in the Swan is the same which has made its appearance within the last few weeks; but on this point the evidence is uncertain.

On April 28, 1848, Mr. Hind (Superintendent of the Nautical Almanac, and discoverer of ten new members of the solar system) noticed a new star of the fifth magnitude in the Serpent-Bearer, but in quite another part of that large constellation than had been occupied by Kepler's star. A few weeks later, it rose to the fourth magnitude. But afterwards its light diminished until it became invisible to ordinary eyesight. It did not vanish utterly, however. It is still visible with telescopic power, shining as a star of the eleventh magnitude, that is, five magnitudes below the faintest star discernible with the unaided eye.

This is the first new star which has been kept in view since its apparent creation. But we are now approaching the time when it was found that as so-called new stars continue in existence long after they have disappeared from view, so also they are not in reality new, but were in existence long before they became visible to the naked eye.

On May 12, 1866, shortly before midnight, Mr. Birmingham, of Tuam, noticed a star of the second magnitude in the Northern Crown, where hitherto no star visible to the naked eye had been known. Dr. Schmidt, of Athens, who had been observing that region of the heavens the same night, was certain that up to 11 P.M., Athens local time, there was no star above the fourth magnitude in the place occupied by the new star. So that, if this negative evidence can be implicitly relied on, the new star must have sprung at least from the fourth, and probably from a much lower magnitude, to the second, in less than three hourseleven o'clock at Athens corresponding to about nine o'clock by Irish railway time. A Mr. Barker, of London, Canada, put forward a claim to having seen the new star as early as May 4—a claim not in the least worth investigating, so far as the credit of first seeing the new star is concerned, but exceedingly important in its bearing on the nature of the outburst affecting the star in Corona. It is unpleasant to have to throw discredit on any definite assertion of facts; unfortunately, however, Mr. Barker, when his claim was challenged, laid before Mr. Stone, of the Greenwich Observatory, records so very definite of observations made on May 4, 8, 9, and 10, that we have no choice but either to admit these observations, or to infer that he fell under the

delusive effects of a very singular trick of memory. He mentions in his letter to Mr. Stone that he had sent full particulars of his observations on those early dates to Professor Watson, of Ann Arbor University, on May 17; but (again unfortunately) instead of leaving that letter to tell its own story in Professor Watson's hands, he asked Professor Watson to return it to him: so that when Mr. Stone very naturally asked Professor Watson to furnish a copy of this important letter, Professor Watson had to reply, ‘About a month ago, Mr. Barker applied to me for this letter, and I returned it to him, as requested, without preserving a copy. I can, however,' he proceeded, 'state positively that he did not mention any actual observation earlier than May 14. He said he thought he had noticed a strange star in the Crown about two weeks before the date of his first observation-May 14-but not particularly, and that he did not recognise it until the 14th. He did not give any date, and did not even seem positive as to identity. . . . When I returned the letter of May 17, I made an endorsement across the first page, in regard to its genuineness, and attached my signature. I regret that I did not preserve a copy of the letter in question; but if the original is produced, it will appear that my recollection of its contents is correct.' I think no one can blame Mr. Stone, if, on the receipt of this letter, he stated that he had not the slightest hesitation' in regarding Mr. Barker's earlier observations as not entitled to the slightest credit.'1

6

It may be fairly taken for granted that the new star leapt very quickly, if not quite suddenly, to its full splendour. Birmingham, as we have seen, was the first to notice it, on May 12. On the

1 Still, a circumstance must be mentioned which tends to show that the star may have been visible a few hours earlier than Dr. Schmidt supposed. Mr. M. Walter, surgeon of the 4th regiment, then stationed in North India, wrote (oddly enough, on May 12, 1867, the first anniversary of Mr. Birmingham's discovery) as follows to Mr. Stone:-'I am certain that this same conflagration was distinctly perceptible here at least six hours earlier. My knowledge of the fact came about in this wise. The night of the 12th of May last year was exceedingly sultry, and about eight o'clock on that evening I got up from the tea-table and rushed into my garden to seek a cooler atmosphere. As my door opens towards the east, the first object that met my view was the Northern Crown. My attention was at once arrested by the sight of a strange star outside the crown' (that is, outside the circlet of stars forming the diadem, not outside the constellation itself). The new star was then certainly quite as brightI rather thought more so-as its neighbour Alphecca,' the chief gem of the Crown. 'I was so much struck with its appearance, that I exclaimed to those indoors, "Why, here is a new comet!" He made a diagram of the constellation, showing the place of the new star correctly. Unfortunately, Mr. Walter does not state why he is so confident, a year after the event, that it was on the 12th of May, and not on the 13th, that he noticed the new star. If he fixed the date only by the star's appearance as a secondmagnitude star, his letter proves nothing; for we know that on the 13th it was still shining as brightly as Alphecca, though on the 14th it was perceptibly fainter.

evening of May 13, Schmidt of Athens discovered it independently, and a few hours later it was noticed by a French engineer named Courbebaisse. Afterwards, Baxendell, of Manchester, and others, independently saw the star. Schmidt, examining Argelander's charts of 324,000 stars (charts which I have had the pleasure of mapping in a single sheet) found that the star was not a new one, but had been set down by Argelander as between the ninth and tenth magnitudes. Referring to Argelander's list, we find that the star had been twice observed-viz., on May 18, 1855, and on March 31, 1856.

Birmingham wrote at once to Mr. Huggins, who, in conjunction with the late Dr. Miller, had been for some time engaged in observing stars and other celestial objects with the spectroscope. These two observers at once directed their telescope armed with spectroscopic adjuncts-the telespectroscope is the pleasing name of the compound instrument-to the newcomer. The result was rather startling. It may be well, however, before describing it, to indicate in a few words the meaning of various kinds of spectroscopic evidence.

The light of the sun, sifted out by the spectroscope, shows all the colours but not all the tints of the rainbow. It is spread out into a large rainbow-tinted streak, but at various places (a few thousand) along the streak there are missing tints; so that in fact the streak is crossed by a multitude of dark lines. We know that these lines are due to the absorptive action of vapours existing in the atmosphere of the sun, and from the position of the lines we can tell what the vapours are. Thus, hydrogen by its absorptive action produces four of the bright lines. The vapour of iron is there, the vapour of sodium, magnesium, and so on. Again, we know that these same vapours, which, by their absorptive action, cut off rays of certain tints, emit light of just those tints. In fact, if the glowing mass of the sun could be suddenly extinguished, leaving his atmosphere in its present intensely heated condition, the light of the faint sun which would thus be left us would give (under spectroscopic scrutiny) those very rays which now seem wanting. There would be a spectrum of multitudinous bright lines, instead of a rainbow-tinted spectrum crossed by multitudinous dark lines. It is, indeed, only by contrast that the dark lines appear dark, just as it is only by contrast that the solar spots seem dark. Not only the penumbra but the umbra of a sunspot, not only the umbra but the nucleus, not only the nucleus but the deeper black which seems to lie at the core of the nucleus, shine really with a lustre far exceeding that of the electric light, though by contrast with the rest of the sun's surface the penumbra looks dark, the umbra darker

still, the nucleus deep black, and the core of the nucleus jet black. So the dark lines across the solar spectrum mark where certain rays are relatively faint, though in reality intensely lustrous. Conceive another change than that just imagined. Conceive the sun's globe to remain as at present, but the atmosphere to be excited to many times its present degree of light and splendour: then would all these dark lines become bright, and the rainbow-tinted background would be dull or even quite dark by contrast. This is not a mere fancy. At times, local disturbances take place in the sun which produce just such a change in certain constituents of the sun's atmosphere, causing the hydrogen, for example, to glow with so intense a heat that, instead of its lines appearing dark, they stand out as bright lines. Occasionally, too, the magnesium in the solar atmosphere (over certain limited regions only, be it remembered) has been known to behave in this manner. It was so during the intensely hot summer of 1872, insomuch that the Italian observer Tacchini, who noticed the phenomenon, attributed to such local overheating of the sun's magnesium vapour the remarkable heat from which we then for a time suffered.

Now, the stars are suns, and the spectrum of a star is simply a miniature of the solar spectrum. Of course, there are characteristic differences. One star has more hydrogen, at least more hydrogen at work absorbing its rays, and thus has the hydrogen lines more strongly marked than they are in the solar spectrum. Another star shows the lines of various metals more conspicuously, showing that the glowing vapours of such elements, iron, copper, mercury, tin, and so forth, either hang more densely in the star's atmosphere than in our sun's, or, being cooler, absorb their special tints more effectively. But speaking generally, a stellar spectrum is like the solar spectrum. There is the rainbow-tinted streak, which implies that the source of light is glowing solid, liquid, or highly compressed vaporous matter, and athwart the streak there are the multitudinous dark lines which imply that around the glowing heart of the star there are envelopes of relatively cool vapours.

We can understand, then, the meaning of the evidence obtained from the new star in the Northern Crown.

In the first place, the new star showed the rainbow-tinted streak crossed by dark lines, which indicated its sun-like nature. But, standing out on that rainbow-tinted streak as on a dark background, were four exceedingly bright lines-lines so bright, though fine, that clearly most of the star's light came from the glowing vapours to which these lines belonged. Three of the lines belonged to hydrogen, the fourth was not identified with any known line.

Let us distinguish between what can certainly be concluded from this remarkable observation, and what can only be inferred with a greater or less degree of probability.

It is absolutely certain that when Messrs. Huggins and Miller made their observation (by which time the new star had faded from the second to the third magnitude), enormous masses of hydrogen around the star were glowing with a heat far more intense than that of the star itself within the hydrogen envelope. It is certain that the increase in the star's light, rendering the star visible which before had been far beyond the range of ordinary eyesight, was due to the abnormal heat of the hydrogen surrounding that remote sun.

But it is not so clear whether the intense glow of the hydrogen was caused by combustion or by intense heat without combustion. The difference between the two causes of increased light is important; because on the opinion we form on this point must depend our opinion as to the probability that our sun may one day experience a similar catastrophe, and also our opinion as to the state of the sun in the Northern Crown, after the outburst. To illustrate the distinction in question, let us take two familiar cases of the emission of light. A burning coal glows with red light, and so does a piece of iron placed in a coal fire. But the coal and the iron are undergoing very different processes. The coal is burning, and will presently be consumed; the iron is not burning (except in the sense that it is burning hot, which means only that it will make any combustible substance burn which is brought into contact with it), and will not be consumed though the coal fire be maintained around it for days and weeks and months. So with the hydrogen flames which play all the time over the surface of our own sun. They are not burning like the hydrogen flames. which are used for the oxyhydrogen lantern. Were the solar hydrogen so burning, the sun would quickly be extinguished. They are simply aglow with intensity of heat, as a mass of red-hot iron is aglow; and, so long as the sun's energies are maintained, the hydrogen around him will glow in this way without being consumed. As the new fires of the star in the Crown died out rapidly, it is possible that in their case there was actual combustion. On the other hand, it is also possible, and perhaps on the whole more probable, that the hydrogen surrounding the star was simply set glowing with increased lustre owing to some cause not as yet ascertained.

Let us see how these two theories have been actually worded by the students of science themselves who have maintained them.

'The sudden blazing forth of this star,' says Mr. Huggins, ‘and