Imágenes de páginas
PDF
EPUB

The 20 feet Newtonian, on Sir W. Herschel's construction, with specula of 18 inches clear aperture (of which three were provided,) was the sheet anchor of the campaign at the Cape. But along with it he carried a 7 feet achromatic by Tulley, with 5 inches aperture-a telescope which had served specially for the measurement of double stars in England, and of the performance of which Sir John gives in his papers in the Astronomical Memoirs a most flattering account, stating even that its performance appeared to improve with each fresh addition of power applied to it.

of the Government Magnetic Observatories | very much more frequently required than in occupied much of his attention,* and within England; and it may be regarded as fortunate a comparatively short time he wrote two that I did not, as at first proposed, (relying on the most excellent and detailed biographies of possession of the three perfect metals,) leave the his astronomical friends, Baily and Bessel. apparatus in question behind. Being apprehensive that in a climate so much warmer, difficulties We may, and must, lament, indeed, that would arise in hitting the proper temper of the time so valuable to science should have been polishing material, slight imperfections of surface, largely spent upon the most mechanical induced by exposure, were for a while tolerated; arithmetical computations connected with but confidence in this respect once restored, and the reductions of places of double stars and practice continually improving, I soon became fastidious, and on the detection of the slightest nebulæ. The author no doubt laments it as dimness on any part of the surface, the metal was much as we do, and informs us (p. 5) that at once remanded to the polisher."-Introd. p. x. he found himself at least unequal to the intended task of going through the whole of these reductions twice; but it appears that he has always found a difficulty, or felt a scruple, in employing an assistant for such operations; which we regret, because we have little doubt that a mere plodding arithmetician would have done the work with as few, if not fewer mistakes; and years might have been added to Sir John Herschel's term of vigorous exertion in the cause of science. The same objection does not, however, apply to the mechanical facility which he happily possesses (in common with his father) of fashioning his own tools and polishing the specula of his telescopes with his own hands. Such dexterity, and such mechanical habits, are of the highest value in themselves to the practical philosopher. They afford a seasonable variety of occupation conducive to mental and bodily health; as he is to employ the instruments, he can scrutinize their defects, and endeavor to remedy them in a way that a person not himself a mechanic might never think of. The very manipulation of such a kind as figuring reflectors will suggest to the ardent and anxious mind of the philosopher, who must devote many hours to it, improvements which might not theoretically occur to him, and which would never occur to an ordinary artisan. But the grand advantage of all is the absolute independence of external assist ance and of skilled workmen which it gives:

"The operation of repolishing was performed whenever needed, the whole of the requisite apparatus being brought for the purpose. It was

* Amongst other efforts to engage public sympathy on behalf of the magnetic cause, Sir J. H. wrote a comprehensive article on the subject in the Quarterly Review, vol. lxvi., p. 271.

In one of his former papers Sir John Herschel, speaking of numerical calculations, says, "for which I find in myself a great inaptitude." (Astr. Soc. Memoirs, vol. v. p. 221.) It is sad to think of the tear and wear of so accomplished a mind exerted in the mere arithmetic of the volume before us. VOL. XVIII. NO. L 6

We shall now give a short analysis of the contents of the volume before us, which is a handsome quarto of 452 well-filled pages, illustrated by 17 plates.

The first chapter is on the NEBULÆ of the Southern Hemisphere. To enter into any detail on this subject would be to discuss a general question of astronomy which could receive no justice within our limits, and a great deal of which is as much connected with other writings of Sir John Herschel and with his father's as with the work before

us.

We have again the highly condensed, almost algebraical language, by which the characters and general effect of nebula have been so graphically described by the father and the son. Many, which are visible both at the Cape and in Europe, are here reobserved; the remainder are either new or "have been identified with more or less certainty with objects observed by Mr. Dunlop, and described in his Catalogue of Nebula." These are 206 in number. The rest of the 629 objects comprised in that catalogue," adds Sir John, "have escaped my observation; and I am not conscious of any such negligence in the act of sweeping as could give rise to such a defalcation; but, on the contrary, by entering them on my working lists (at least until the general inutility of doing so, and loss of valuable time in fruitless search thereby caused, became apparent) took

[ocr errors]

the usual precautions to ensure their discovery."

Here is a sad tale and warning: for errors like Mr. Dunlop's not only deprive the more conscientious labors of their author of almost all their value, but they inflict a grave and positive injury upon the science which they pretend to promote. If men like Herschel are to spend the best years of their lives in recording for the benefit of a remote posterity the actual state of the heavens, in order that their changes may be examined and pronounced upon, what a galling discovery to find amongst their own contemporaries men who, without any wish to invent (we do not mean to charge Mr. Dunlop with that,) but merely from carelessness and culpable apathy hand down to posterity a mass of errors, bearing all the external semblance of truth; a quintessence of error so refined, that four hundred objects out of six hundred could not be identified in any manner, after only eight years, by the first observer of the day, and with a telescope seven times more powerful than that stated to have been used! We can add nothing to an exposure so humiliating.

Sir John's chapter on Nebula contains several distinct sections. It would have added to convenience of reference as well as

given a more just idea of the variety and quantity of matter in the volume, had the Table of Contents of the volume been more full.* There is, in the first place, a catalogue of nebulæ and clusters of stars-1708 in number-chiefly in the southern hemisphere, which forms a sequel to the similar catalogue, by the same author, of 2307 objects of the same kind visible in England and published in the Philosophical Transactions for 1383. There is complete symmetry in the mode of description and registration. The descriptions (in abbreviated terms) have reference to Brightness, Size, Form; relation to neighboring Stars; and more particularly to the degree of Condensation of the seeming nebulous matter-a point of much delicacy and difficulty of description, but of capital importance with reference to Sir William Herschel's theory of progressive condensation of rare into dense nebulae, and finally into planetary nebula, nebulous stars, or even clusters of stars. Here is a pretty

[blocks in formation]
[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small]

66

Very faint; round; a little brighter in the middle; 20" in diameter."

The descriptions seem diametrically opposed. Such is the effect of difference of climate at Slough and Feldhausen. But if this be the case--if this be the effect of atmospheric influence (and such Sir John warns us, page 3, that it is) upon observations of the same object by the same telescope, and, within a few years, by the same eye, can we hope to perpetuate descriptions which shall enable posterity to decide upon real changes of physical constitution?

In gen

Sir John gives more particular descriptions of some more remarkable objects. eral we may observe that his figures show less tendency to striking symmetry of form than some of those in his former catalogue ; and it is now not denied that that symmetry was in some cases the involuntary deduction arising from a previous impression in favor of symmetric forms (as in the dumb-bell nebula and the well-known No. 51 of Messier's catalogue.) But the most interesting observations are upon

the nebula in the sword-handle

of Orion, the star Argus, and the Magellanic clouds. Of the former, Sir John gives, which in all probability will be admitted by in Plate VIII., an exquisite representation, astronomers generally to be the most careful delineation of a celestial object ever transferred to copper. There are, perhaps, not ten persons alive in a position to judge of its minute accuracy; but this it will occur to no

one to doubt who has read the present chapter and the paper on this nebula in the "Astronomical Memoirs" of 1824 by the same author. The total want of symmetry of the whole; the sometimes sudden, sometimes infinitely graduated shading off of the misty light, resembling slightly the exquisite shading of a snowy surface tossed into fantastic forms by eddies of wind, rising here and there into seeming ridges, elsewhere into gently swelling domes, or depressed into troughs and basins with cusped boundaries; sometimes apparently representing flats of extensive uniformity, or again mottled in an indescribable manner, as with the touch of the minature-painter's brush--these varieties are well brought out in this magnificent engraving. If we compare it with Sir J. Herschel's older one in the "Astronomical Memoirs," we find such a marked difference in the general character of the two that, though it is easy to see that they are representations of the same object, it appears to throw doubt (as we have already noticed) on the possibility of determining with sufficient exactness the features of such complex and ill-defined objects at one time, to give confidence to our belief of real changes at a future and distant one. Sir J. Herschel gives a hesitating expression of opinion that some of the diversities of the two drawings may be due to a nebular variation in thirteen years (p. 31); but such a conclusion would require strong evidence to support it.

Of Argus, Sir J. Herschel observes: “There is, perhaps, no other sidereal object which unites more points of interest than this. Its situation is very remarkable, being in the midst of one of those rich and brilliant masses, a succession of which, curiously contrasted with dark adjacent spaces, (called by the old navigators coal-sacks,) constitute the Milky-way in that por

tion of its course which lies between the Centaur

and the main body of Argo. In all this region the stars of the Milky-way are well separated, and except within the limits of the nebula, on a perfectly dark ground, and, on an average, of larger magnitude than in most other regions. In two hours, during which the area of the heavens swept over consisted of 47:03 degrees, the amazing number of 147,500 stars must have passed under review. In the midst of this vast stratum of stars occurs the bright star of 7 Argûs, an object in itself of no ordinary interest, on account of the singular changes its lustre has undergone within the period of authentic astronomy." -p. 33.

Sir John then goes on to state that by Halley (in 1677) Argûs was marked as of the fourth magnitude; in Lacaille's and

[ocr errors]

later catalogues it is denoted by the second; and as observed by himself, from 1834 to 1837, was counted as a large star of the second, or small one of the first magnitude. 'It was on the 16th of December, 1837," he adds, "that my astonishment was excited by the appearance of a new candidate for distinction among the very brightest stars of the first magnitude." This was his old acquaintance Argus. "Its light was, however, nearly tripled!" About the 2d of January, 1838, its light was judged to be a maximum, and all but equal to that of the very bright star a Centauri; but it had manifestly fallen below that on the 20th of the same month. At the conclusion of Sir John's personal observations, in April, 1838, it had "so far faded as to bear comparison with Aldebaran, though still somewhat brighter than that star

"Beyond this date I am unable to speak of its further changes from personal observation. It appears, however, since that period to have made another and still greater step in advance, and to have surpassed Canopus, and even to have approached Sirius in lustre, the former of which stars I estimate at double, the latter at more than quadruple of a Centauri, so that Jupiter and Venus may possibly have a rival amongst the fixed stars in Argo, as they have on recorded occasions had in Cassiopeia, Serpentarius, and Aquila."—p.

34.

[blocks in formation]

gellanic clouds, serve to give us the highest | give a first approximation to the orbits and idea of the indomitable patience of Sir J. Herschel as an observer. There are two sections attached to this chapter-one on the Law of Distribution of Nebula and Clusters of Stars over the Surface of the Heavens, the other on the Classification of Nebula, which presents some interesting general remarks:

"The distribution of nebulæ is not, like that of the Milky-way, in a zone or band encircling the heavens; or, if such a zone can be traced out, it is with so many interruptions, and so faintly marked out through by far the greater part of its circumference, that its existence as such can be hardly more than suspected. One-third of the whole nebulous contents of the heavens are included in a broad, irregular patch, occupying about one-eighth of the whole surface of the sphere, chiefly (indeed almost entirely) situated in the northern hemisphere, and occupying the constellations Leo, Leo Minor, the body, tail, and hind legs of Ursa Major, the nose of the Camelopard, and the point of the tail of Draco, Canes Venatici, Coma, the preceding leg of Bootes, and the head, wings, and shoulders of Virgo. This, for distinction, I shall call the nebulous region of Virgo."-p. 134.

The chapter concludes with a detailed description of the two Magellanic clouds, or nebulous regions, in which (with his accustomed perseverance) Sir J. Herschel has determined the positions of a vast number of individual stars, which he has made subservient to the construction of a general chart of the greater cloud in Plate X. of his work. The second chapter is devoted to the subject of DOUBLE STARS. The great interest of these observations is altogether prospective. Sir John has now done for the Southern Hemisphere what his father commenced in the Northern more than half a century before; that is to say, he determined the existence and marked the relative position of many pairs of stars, which might afterwards prove to be not merely optically double, or seen by the effect of perspective nearly in the same direction, but physically double, that is, really in each other's neighborhood (relatively speaking); and in the circumstances of a planet and satellite, one circulating under the law of gravitation round the other, or, to speak more correctly, both circulating round their common centre of gravity. With only one or two exceptions (such as a Crucis and a Centauri), Sir J. Herschel found no previous observations of old date upon double stars not visible in Europe, which, combined with his own, might

periods of this highly interesting class of bodies. The accurate Lacaille visited the Cape before such observations were attended to; and Mr. Dunlop's Paramatta Catalogue of 253 Double Stars (Mem. Astr. Society, vol. iii.) appears to be little more worthy of confidence than this Catalogue of Nebula. Even the few years which elapsed between the period of Mr. Dunlop's first observations and those of Sir J. Herschel would have sufficed to give a first approximation to the orbits of the faster moving of these twin-suns. But Dunlop, through negligence, indolence, or something worse, has failed to be the elder Herschel of Antarctic Astronomy. The discrepancies are so great and frequent, that we can have scarcely any confidence in those whose agreement with the recent observations is sufficient to allow us to suppose that they might possibly be correct. It must have been disheartening to Sir J. Herschel to put down such a judgment as this: "A great many mistakes appear to have been committed in the catalogue alluded to (Dunlop's), either in the places, descriptions, or measures of the objects set down in it." p. 167. Again, "It is useless reasoning on such hypothetical data," (Dunlop's Angles of Position), p.

288.

Sir John has two catalogues of double stars. The first contains 2102 such objects, observed and placed by the 20-feet reflector, with the angles of position, and a rough guess of their distances. The second contains accurate measures of the distances of the more interesting objects, and also of their angles of position by means of the 7-feet achromatic. There are appended some very interesting" special remarks on the measures of particular double stars in the foregoing_catalogues." "With the two exceptions already referred to, no double star not visible in Europe can be said to have its orbital motion even roughly ascertained by these observations. But there will be a great harvest to be reaped some 20 or 30 years hence, when the objects in the Herschel Catalogue shall be re-examined by some equally conscientious observer.

There is one discussion introduced here too interesting to be passed over-it is as to the orbit of 7 Virginis, a double star on the confines of the two hemispheres, and therefore observable in either. This discussion (p. 291 et seq.) is a continuation of one by Sir J. Herschel in 1832, printed in the 5th vol. of the Memoirs of the Astronomical Society, as an example of a new method of dis

covering the form and position of the orbits a few years after the latter date the variaof double stars from observation. In that tion both of position and distance became topaper he deduced, by peculiar methods, the tally irreconcilable with the old ellipse, and elements of the orbit from 19 observations, a new orbit was first computed by the Gerpartly of position and partly of distance, since man astronomer Mädler,* which has its ma1780; he included also two older observa-jor axis almost at right angles with the fortions by Bradley and Mayer, in 1718 and mer one, and an area 11 times smaller. 1756, and the whole appeared to be quite sufficiently satisfied by supposing the one star to revolve round the other in 513 years, in an orbit having a major semi-axis (as seen from the earth) subtending 1183. He also made (in 1832) this prediction: "The latter end of the year 1833 or beginning of the year 1834 will witness one of the most striking phenomena which sidereal astronomy has yet afforded, viz., the perihelion passage of one star round another, with the immense angular velocity of between 60° and 70° per annum, that is to say, of a degree in five days." This occurrence actually took place during Sir John's residence at the Cape, though not exactly at the predicted time, but rather towards the middle of 1836, for some time before and after which the appulse of the two stars was so close, that even in the 20-feet reflector, under the sky of the Cape, and by the eye of Herschel, they could not be divided.

Sir J. Herschel, with his usual candor, does not attempt to gloss over the error into which he had fallen. The error was quite natural, and the remark he makes is most just, namely, that "this is not the first by many instances in the history of scientific progress, where, of two possible courses, each at the moment equally plausible, the wrong has been chosen." Sir John's final result is an orbit described in 182 years, with a major semi-axis of only 3'58. But other astronomers are of opinion that a period of about 143 years is the true one. Mädler and Henderson were of this opinion, which shows that some uncertainty still exists-an uncertainty inherent in the problem, since both hypotheses satisfy the observations fairly, as may be seen by comparing Sir J. Herschel's Table of Calculated and Observed Places with Professor Henderson's in Captain Smyth's Cycle of Celestial Objects, vol. i. p. 486. A good deal depends on the choice of observations to be satisfied; those by different astronomers, and particularly by the elder Struve, appearing to have peculiar and constant sources of error.

But there is a circumstance purely geometrical which creates great ambiguity. The inclination of the plane of the real elliptic orbit (for, throughout, the conformity of the elliptic motion to the law of gravity is assumed) to the radius of vision or to the ideal concave surface of the celestial sphere, is absolutely unknown à priori. But though an ellipse seen obliquely always appears as an ellipse, the position of the focus (the principal or central star) may be totally distorted by the effect of perspective; and as the law of the equable description of areas will also hold in the distorted ellipse, we are wholly

The elements of 1832 did not, however, long satisfy the requirements of this quickly moving star. Next year Sir John modified them, increasing the period to 629 years and the major semi-axis to 12"09. The comparison of the new elements with the observations from 1718 to 1833 agreed, as he stated, "so well throughout the whole series as to leave nothing to desire." What a lesson this to physical philosophers in drawing conclusions! So far from leaving nothing to desire, these elements, with the exception of the eccentricity, had little or no resemblance to the true elements of the apparent orbit; and the revolving star, instead of describing only about one-fifth of its ellipse in 115 years during which it had been observed, had in reality completed two-thirds of its period, perhaps more. To understand how this could possibly happen, we must refer to the interesting diagram, p. 293 of the work before us, which shows the true ellipse nesFontenelle, we think, adds that the least protled so snugly into one end of the former hy-similar, but less justifiable, mistake occurs in Probable is commonly the true one. A curious and pothetical orbit intersecting it in four points, fessor Playfair's estimate of the shortest time rethat they nearly coincide for a large portion quired by a heavy body to describe the slide of of the smaller orbit, and precisely that por- Alpnach, supposing it a cycloid, which he makes tion described between 1718 and 1833; but about a fourth part too small. But it is just to recollect contrary instances, when they do occur, showing that fate is not always adverse to the bold inquirer. Of this several circumstances in the recent discovery of Neptune offer striking instances.

* Mem. Astr. Society, v 194.
† Ibid., vi. p. 152.

* Astronomische Nachrichten, No. 363, for 1838,

and No. 452. for 1842.

« AnteriorContinuar »