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"I will come," he said. Ah! Love, come With russet freckles on his face

66

From The Nineteenth Century.
RECENT SCIENCE.

I.

of the group, and on examining the whole, one cannot refrain from concluding that the stars are simply spots upon which the diffuse nebulous matter has agglomerated and condensed to make new suns. The same is also seen in the photographs of the nebulæ in Orion - the more so as the spectroscope reveals the unity of compowhich surround them and link them together.

sition of both the stars and the nebulæ

A BREATH of youthful energy and youthful hopes inspires modern astronomical work. Astronomy, the oldest of the sciences, has more than renewed her youth," as William Huggins said at the end of the inaugural address he delivered before the last meeting of the British Association. Since the spectroscope, formerly used but to study and reveal the obtained by H. C. Russell with his photoStill more interesting results have been chemical composition of the celestial bodies, has become an instrument for graphs of nebulæ in the constellation of Argus. His earlier photographs, obtained measuring their unseen movements and by a three-hours' exposure, have already for penetrating into the secrets of their been referred to with admiration by Wilhistory, and since photography has been taken as a necessary auxiliary by astron the photographic film was exposed for liam Huggins in his address. But when omers, a new chapter of astro-physics has been opened. The proper movements of eight hours to the faint light of the nebula, the stars have acquired a new meaning; new facts were revealed. The photograph the faint masses of nebulous matter, scatnot only shows that the nebulous matter tered round and amidst the stars, have extends far beyond the limits assigned to become animated indications of the gene-servations at the Cape, while confirming it by Herschel during his memorable obsis of solar systems; and the great prob- at the same time the great accuracy of the lems relative to the life of the stellar description of what he did see; it also worlds- their origin, their growth, their decay, and their rejuvenescence- have proves that the nebula has lived since 1837, and has altered considerably its come again to the front, supported by renewed hopes as to the proximity of their aspect during the last fifty years. At the very same place where Herschel saw one of its brightest and most conspicuous parts, we have now a dark oval space, upon

ultimate solution.

It is not possible, indeed, to examine the splendid photographs, made by Mr. Roberts, of the nebula in Andromeda, and to see this whirlpool of luminous matter, divided into dark and bright rings surrounding a large, undefined central mass, without perceiving in it a gigantic solar system in the way of formation, and without concluding in favor of a similar origin, on a much smaller scale, of our own solar system. The best drawings of the same nebula, which were made by Bond and John Herschel with the aid of the best telescopes, told nothing of the kind; the complicated structure of the nebula, its life, were missing in what was reproduced by the pen of a cautious observer.

Again, in another part of the sky-the Pleiades - the photographs of the Brothers Henry show at once that this cluster of suns is not an occasional gathering. Streaks of nebulous matter, revealed by photography, connect together the stars

which no trace of luminous matter can be drawn elsewhere, or is luminous no more; detected. The matter either has been may be, it is passing through some stage preparatory to the appearance of a new star. We are thus convinced that these tic their dimensions, are living at a much accumulations of matter, however giganmore rapid speed than we were prepared to admit. Changes occur in them, even within the short limits of one man's life; and as the new star in Auriga, rapidly passing through a series of transforma birth of new suns,* so also we may hope tions, reveals to us the secrets of the that the study of the modifications of the

nebulæ will initiate us into the secrets of

the earlier stages of development of the

stellar worlds. In the movements of those

See an article by Mr. Norman Lockyer in LIVING AGE, No. 2497, p. 323.

remote agglomerations we learn to feel | The spectra of the stars, the nebulæ, the

the continuous life of nature, its continuous change, its evolution.

corona, and the protuberances of the sun, are now photographed; and by this means When the great photographic map of the powers of the astronomer are considerthe whole sky is ready, many a change in ably extended. He can study the specthe stellar worlds and nebulæ which es- trum in its ultra-violet part, which is not capes now our attention will be recorded visible to the eye, as it hardly acts upon forever. The preparatory work is already our retina, while its chemical rays act completed; the instruments are chosen, very well upon the photographic sensitive and the uniformity of methods is secured. plate; he obtains greater enlargements of The sky is apportioned between the eigh- the spectrum, and he can study the spectra teen observatories which will perform the at his leisure and measure the positions of whole of this immense work, each of them the bright or dark lines which intersect having to make from one thousand to fif- them—the more so as the spectrum of teen hundred separate photographs in some well-known body (incandescent hy order to map all stars down to the sixteenth drogen or iron) is photographed on the magnitude; and the first specimens already same plate for the sake of comparison. published satisfy the most severe exigen- This method has already given some excies of the astronomers. Many new facts cellent results. It has permitted us to are sure to be revealed by this grand sur-measure the movements of the stars in vey of the sky, because even now, when a the line of vision with a quite unexpected simple preliminary exploration is being accuracy. The proper movements of the made, we can already mention some dis- stars offer an immense interest; but while coveries due to photography. Thus, when we always could ascertain their movethe amateur astronomer, Dr. Anderson ments north and south, or west and east, (equipped with but a small pocket tele- on the celestial sphere, we formerly had scope and the little atlas of the sky by no means of telling whether a star is apKlein), discovered on the 31st of January proaching us, or going away, during its the new star in Auriga, it appeared that displacements in space. The spectrothe newcomer had already been photo-scope gives those means. graphed without astronomers being aware of the fact. Professor Pickering found its portrait on photographs taken on three different occasions since the Ist of December, and the indefatigable Heidelberg astronomer, Max Wolf, also had it on his photographs since the 8th of the same month. The appearance of the new star thus would have been recorded, even if nobody had remarked its appearance. Another photographic discovery is due to the same Max Wolf. Having photographed one part of the sky on two consecutive nights in December, he sent his negatives to Dr. Berberich, who at once noticed that two minute spots had changed their positions in the twenty-four hours. One of them proved to be a new addition to the list of minor planets, while the other was a previously known small planet of the same group.

However, the chief progress recently achieved in physical astronomy is due to the spectroscope aided by photography.

The spectrum

of a star usually consists of a band of faint
light, intersected by several bright (or
dark) lines, corresponding to the lines ap-
pearing in the spectra of hydrogen, cal-
cium, iron, magnesium, natrium, and so
on. But if we reproduce under the spec-
trum of the star the spectrum of, say,
hydrogen, we often see that the hydrogen
lines in the former do not quite coincide
with the same lines of the latter; they
are slightly displaced to the right or to the
left. William Huggins long ago explained
that this displacement is due to the proper
movements of the stars and gives a means
of measuring them, and Mr. Christie even
measured in this way, several years ago,
the otherwise invisible movements of sev-
eral stars. In fact, the blue and violet
light of the spectrum is due to very quick,
luminous vibrations, while its red light is
I due to much slower vibrations, just as the
high pitch of a sound depends on much
quicker vibrations of the air than the low
pitch. But if a star approaches us with a

t

the Earth at a speed of 7.4 miles in a second; and when we determine the same speed with the aid of the spectroscope, we find 7.8 miles. The spectroscope errs by but four-tenths of a mile- by less than seven hundred yards!*

great rapidity, our eye will receive from it | tance. We may calculate beforehand tha more vibrations in a second, and its light at a given moment Venus will approach will appear bluer, so to say; in other words, its spectral bright lines will be shifted towards the blue end of its spectrum; and they will be shifted towards the red end if the star goes away with the same rapidity. In our century of railways many of us must have witnessed an analogous fact when looking at an express train passing by a station. When the rapidly running engine sounds its whistle, the pitch of the whistle seems to become higher as the train approaches us, and it seems to become lower when it goes away - the ear receiving in a second of time more and more vibrations in the former case, and less vibrations in the second case. So it is also with the stars, and the advantages of having the spectrum of the star and the comparison spectrum photographed on the same plate are self-evident.

If we examine, for instance, the photographed spectra of Sirius we see that their hydrogen lines are always shifted towards the blue end of the spectrum, and from this we may safely conclude that the star is approaching us. And if we calculate the speed of its approach, we find it (after having taken into account the movement of the earth in its orbit) to be about seven miles in a second. The measurements may be made at different observatories and at different seasons of the year; the final results will not differ from each other by more than one mile, or even a fraction of a mile. We do not know the immense distance which separates us from Sirius, we only gauge it by saying that its light takes nearly sixteen and a half years to reach us; but a change of seven miles per second in that enormous distance is revealed by the spectrum. These results seem almost incredible, and they could not be relied upon had they not been submitted to severe tests. Thus we know the movements of the earth in its orbit, and we conclude that they must be reflected in our measurements, if these measurements are sufficiently accurate; and they are reflected with perfect accuracy. Again, we know the distance which separates us from Venus, and how the movements of both the Earth and Venus affect this dis

We may thus place full confidence in our new auxiliaries. When Mrs. Flemming and Miss Maury, on examining the spectrum of ẞ Lyræ, remarked that it consists in reality of two spectra periodically superposed, and Professor Pickering concluded therefrom that the star must consist of two luminous bodies which rotate around a common centre of gravity at a very great speed,f or when we are told that the new Auriga star consists of at least three separate agglomerations of incandescent gases, we can safely rely upon these conclusions.

And, finally, the spectroscope, combined with photography, enables us to explore the ultra-violet part of the spectrum quite invisible to the eye. By using this method, Hale at Chicago, and Deslandres at Paris, obtain day by day the positions of those solar emissions of incandescent gas, or protuberances, which consist chiefly of incandescent hydrogen, and the light of which is so feeble that they escape observation, even during the eclipses of the sun, when its light is screened by the moon. The movements of these invisible clouds are now studied like the movements of our own atmosphere, and we learn that the laws of cyclonic storms which prevail on the earth hold good for the hot vapors of hydrogen and calcium on the surface of the sun. The unity of Nature and her laws thus receives a further brilliant confirmation.

II.

ANOTHER question which, although it has a direct bearing upon our own terrestrial affairs, preoccupies astronomers considerably, is the variation of latitudes.

Prof. Vogel at the Astronomical Society (Observatory, January, 1892).

Observatory, October, 1891.

Comptes Rendus de l'Académie des Sciences, 1891, t. 113, p. 307.