SIR JOHN HERSCHEL'S ASTRONOMICAL OBSERVATIONS AT THE CAPE.
THE sojourn of Sir John Herschel at the Cape of Good Hope forms a memorable epoch in modern astronomy, not merely for the particular
discoveries or observations which resulted from his visit, but as com- pleting that "telescopic survey of the visible heavens" which he had commenced in 1825 at home, and finished after a labour of fourteen years. As this task carries us back to "the good old times" when learned labour was performed as a matter of conscience and life was de-
voted to a set duty, so the present appearance of the conscience, of Astro- nomical Observations recalls the age of princely patronage' when in-
dividuals of large possessions applied some of their wealth to do that for art or letters which could not be done otherwise. As a matter of trading speculation, this expensive quarto could never have been published : the demand for it must be far too limited to justify the expectation of a re- turn even to a moderate proportion of the outlay; and but for the muni- ficence of the house of Northumberland the contents of the volume would have been scattered. "To the munificent destination of his Grace the late Duke of Northumberland of a large sum in aid of its publication," Sir John Herschel writes, "it owes its appearance as a single and sepa- rate work, instead of a series of unconnected memoirs, scattered over the volumes of academical bodies. The lamented decease of that illustrious nobleman [to whose memory the volume is dedicated] prevented his wit- nessing its final completion. His liberal intentions, however, have been fully carried out by the worthy successor to his title and his spirit : whose kind and gracious interest in it I should be wanting in all proper feeling were I to omit this opportunity of acknowledging." The first object of the work is to exhibit the heavenly statistics of the Southern Hemisphere; and this important feature consists of catalogues whose facts may be exhibited tabularly : so that, unintelligible as the lists may look to the uneducated eye, they are the product of labour of the most tasking, watchful, and accurate kind, and convey to the astronomer in a species of scientific short-hand the results of nights of toilsome observa- tion and days of calculation, being to him what charts are to a mariner. The larger subjects thus presented are the statistical distribution of stars and the constitution of the Galaxy in the Southern Hemisphere ; the nebula
and double stars of the same region of the heavens ; and an interesting chapter on "astrometry," or an endeavour to fix, by means of the use
of figures, the apparent magnitudes of the stars. More particular ques- tions are also treated : circumstances, or the position and climate of the Cape, enabled Herschel to observe Halley's Comet to peculiar advantage, as well as the Satellites of Saturn, and the Solar Spots during part of 1836 and 1837; and very valuable contributions on each of the three subjects are the result.
In stating that heavenly statistics, or information condensed and tabularly presentable, are the first results of the observations at the Cape, we do not mean that they are the only features of the work. Every table has remarks appended to each item, sometimes in astronomical hieroglyphics, sometimes of a more descriptive kind : each of the seven chapters is prefaced by an elaborate paper, discussing various points con- nected with the subject, noticing many exceptional or dubious facts, and incidentally furnishing sketches of the story of the observations, which combine an interest similar to that of autobiography and travels. We see the difficulty the astronomer had to contend with from the state of the weather, intercepting or obscuring the heavens ; the occasional failure of instruments, sometimes overcome by persevering ingenuity, at other times rendering the perfect accuracy of the observations incomplete ; while the accounts of the manner in which particular operations were carried on place us behind the scenes, and show how much of physical and painful labour the astronomer must undergo, in addition to minute observation and incessant watchfulness.
The scientific results of this volume, technically stated, are not adapted to a miscellaneous journal : common readers would not follow them; students will seek them, in a complete form, in the work itself Even the popular deductions that might be drawn from this elabo- rate quarto would trench too much upon space. But we note a point or two, of sufficient interest for newspaper perusal. Throughout the volume, the filial veneration of Sir John Herschel is conspicuous, and sometimes, from the peculiarity of a recurring phrase, has an air of simple family bonhommie, that inspires regard while it raises a
smile. Into the nebular hypothesis of "my father" Sir John does not directly enter, though he seems to consider it unshaken and there are Clearly nebulte not resolvable by any present power. That men like the elder Herschel, and other astronomers of high celebrity, should have erred in confounding the "nebulous haze" of the thllowing passage with the ne- bulous substance, is out of the question. Yet this description alone seems to bear upon the case ; for the resolution of nebulte into stars by Lord Itosse's great telescope does not meet the problem. The elder Herschel long supposed that the nebulous matter he encountered consisted of stars, though his telescopic powers could not resolve them. It was the exist- ence of a star in the centre of the nebulous matter which gave rise to his hypothesis, from the obvious question, and its conclusion, "If the nebu- lous appearance consists of irresolvable stars, what can this visible star be ?"
"Among the irregular and accidental optical effects of peculiar atmospheric conditions incidental to the climate, [of the Cape,] there are one or two which seem deserving of especial notice. The first is that phenomenon which, when it occurs, I have designated by the epithet the " nebulous haze." Its effect is to convert every star of the 9th magnitude and upwards into a " nebulous star,"--- meaning thereby a well-defined star, with a faint, nebulous photosphere, of greater
or less extent according to the brightness of the star, surrounding it. This phenomeon occurs in a perfectly clear sky, free from the slightest suspicion of
cloud. It comes on very suddenly and unexpectedly, and goes off as suddenly,
lasting sometimes only a few minutes; at others, longer. Thus, in sweep 500, October 5, 1834, it commenced at 22" 4e, ST, when a star 7 m was observed to
be surrounded with it, having come on quite suddenly, and continued to affect all
the brighter stars until 22" 54,e, when it was quite gone; being described as extraordinary in intensity, and very troublesome daring its continuance. From this time till Oh 27m, all was clear, when it suddenly came on again, 'in an in- stant. A star 7 m was quite free, but on drawing it back' (after it had left the field for reexamination,) 'it was found to be completely involved,' the sky con tinning all the while pure, so far as the naked eye could discern. Again, in sweep 598, June 18, 1835, we have 15h 37e. ST. A nebulous haze came on in an in
stout, extending to stars 9 m; yet the sky is as clear as ever, and the calm nil
broken.—' 15h 44.,—(a star 6 m in the field.) The nebulous haze is gone; it did not last two minutes.'—' 16h 23ei. The nebulous haze come on again in a moment.' Such remarks might lead to a suspicion of dew upon the eye-piece, or the breath of the observer settling on the glass; but repeated examination (the phenomenon being very common) has satisfied me that such is not the cause, but that it is really of atmospheric origin. Similar nebulous affections occur in our English climate; but it is their much greater frequency, and the suddenness of their appearance and disappearance, which form so remarkable a feature at the Cape."
The more striking objects are illustrated by plates. One of these is devoted to n Argus, and the great nebula surrounding it. "There is
perhaps," says Sir John Herschel, "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 naviga- tors coal-sacks,) constitute the Milky Way in that portion 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 magnitudes than in most other regions." Some idea of the number of stars may be formed from the fact that in two hours, "during: which the area of the heavens swept over consisted of 3° x 300 x sin. 1480 30' =47.03 square degrees, the amazing number of 147,500 stars must have passed under review."
It is not these circumstances, however, that induced us to recur to n Argus ; the singular changes it has undergone and is undergoing form its most appreciably wonderful feature.
"In the midst of this vast stratum of stars occurs the bright star Argus, an object in itself of no ordinary interest on account of the singular changes its lustre has undergone within the period of authentic astronomy. For while in Halley's Cata- logue, (constructed in 1677,) which is the first which can be entirely depended upon, it is marked as of the 4th magnitude, yet in Lacaille's and the subsequent Cata-
logues of Brisbane, Johnson, Fellows, and Taylor, it is made to rank as of the se- cond. When first observed by myself in 183 it appeared as a very large star of the second magnitude, or a very small one of the first; and so it remained without
apparent increase or change up to nearly the end of 1837, in November of which year it was noticed of its usual brightness, or at least without exciting any sus- picion of a change. Nor had any such suspicion been excited during a series of
photometric comparisons set on foot in the beginning of 1836, and carried on when- ever fitting opportunities occurred, with the express object of establishing a scale of Southern magnitudes, and in which this star had been frequently compared with others both superior andinferior to it in brightness. •
"It was on the 16th December 1837, that, resuming the photometrical com- parisons in question, in which, according to regular practice, the brightest stars in sight in whatever part of the heavens were first noticed and arranged on a list, my astonishment was excited by the appearance of a new candidate for distinction among the very brightest stars of the first magnitude, in a part of the heavens with which being perfectly familiar, I was certain that no such brilliant object had
before been seen. After a momentary hesitation, the natural consequence of a phenomenon so utterly unexpected, and referring to a map for its configurations with
the other conspicuous stars in the neighbourhood, I became satisfied of its identity
with my old acquaintance s Argus. Its light was, however, nearly tripled. While yet low it equalled Rigel, and when it had attained some altitude was decidedly
greater. It was far superior to Achernar. Fomalhaut and a Grais were at the
time not quite so high, and a Crucis much lower; but all were fine and clear, ancl ti Argus would not bear to be lowered to their standard. It very decidedly sur- passed Procyon, which was about the same altitude, and was far superior to Alde- baran. It exceeded a Orionis; and the only star (Sirius and Canopus excepted) which could at all be compared with it was Rigel, which, as 1 have stated already, it somewhat surpassed. "From this time its light continued to increase. On the 28th December it was far superior to Rigel, and could only be compared with a Centauri, which it equal- led, having the advantage of altitude, but fell somewhat short of it as the alti-
tudes approached equality. The maximum of brightness seems to have been ob-
tained about the 2d January 1838; on which night, both stars being high and the sky clear and pure, it was judged to be very nearly matched indeed with a Cen- tauri, sometimes the one, sometimes the other being judged brighter, but on the whole a was considered to have some little superiority. After this the light be- gan to fade. Already on the 7th, 13th January, a Centauri was unhesitatingly placed above, and Rigel as unhesitatingly below it. On the 20th, it was 'visibly diminished—now much less than a Centauri and not much greater than Rigel The change is palpable.' And on the 22d, Arcturus, (the nearest star in light and colour to a Centauri which the heavens afford?) when only 100 high, surpassed 'I, the latter being on the meridian; s was still, however, superior to (3 Centauri, a Crucis, and Spica; and continued so (and even superior to Rigel) during the whole of February; nor was it until the 14th April 1838, that it had so far faded as to bear comparison with Aldebaran, though still somewhat brighter than that star.
"Beyond this date Iran unable to speak of its farther changes from personal observation. It appears, however, since that time to have made another and a still greater step in advance, and to have surpassed Canopus, and even to have approached Sirius in lustre."