M. O. Struve proceeds to mention in detail four parts of the nebula of Orion in which he perceived most distinctly, in an interval of some months, changes of form or of the degree of light. The first is a bay, extending from the straits of Le Gentil in the direction of the trapezium of stars situated towards the middle of the nebula. This bay appeared to him at one time altogether obscure, like the straits; at another, full of nebulosity, and little inferior in brightness to the surrounding portions of the region of Huygens. Dr. Lamont first delineated this bay, which has never been seen by Sir John Herschel. The second is a nebulous bridge, which crosses the great straits, with a point of concentrated light about midway. M. Struve saw it in winter, sometimes as represented by Herschel, sometimes as by Liapounoff, with much greater concentration of light, but always much more extended than in the representations of these astronomers, and closely approaching the southern limit of the great strait. Very faint traces of it are indicated by M. Lamont, while Professor Bond did not see it at all. The third is a nebulosity surrounding star 75 of Herschel's catalogue, which appeared to M. Struve to be subject to great changes of brightness. Lastly, the fourth part is a sort of narrow canal, uniting in a right line the obscure space situated around the stars 76, SO, and 84, of Herschel's catalogue, with the north side of the great strait, near the exterior extremity of the bridge before mentioned. The canal, which has not been represented by any other observer, was distinctly seen by M. Struve March 24, 1857, while on other occasions he has not perceived the least trace of it. This astronomer, in closing his communication, adds, that the general impression resulting from his observations is to the effect that the central part of the nebula of Orion is in a state of continual change of brightness as regards many of its portions. In those cases where the images were most distinct, their appearance did not seem entirely uniform from night to night. These changes in the degree of light cannot, however, be perceived in the greater number of cases without instruments of considerable optical power; and he does not think that achromatic telescopes of less than ten inches opening can serve to verify them, except under atmospheric conditions extraordinarily favorable. The twenty-second volume of the M. N. (pp. 203-207) contains the analysis of another memoir relating to the same nebula. It was communicated to the Astronomical Society, May 10, 1861, by Professor George Bond, who has succeeded his father in the direction of the observatory of Harvard College, at Cambridge, near Boston. The paper bears for its title, On the spiral structure of the great nebula of Orion. Professor Bond the father, in a memoir published in 1848, had already remarked that the light of this nebula seemed to present a radiated appearance on its southern side, starting from the neighborhood of the trapezium of stars situated towards its middle. Professor G. Bond has undertaken, since 1857, to form a catalogue of the stars comprised in a square of forty minutes to the side, having 0 of Orion for its centre. He selected one hundred and twenty-one bright stars as guiding points to which to refer the smaller stars, of too feeble light, for the most part, to remain visible under a strong illumination of the micrometric threads. In a first sheet he has arranged two hundred and sixtytwo stars, and then subdivided the same surface into four charts, finally reunited into a single one. The form and arrangement of the clongated luminous tufts, alternating with the more obscure spaces stretching from the neighborhood of the trapezium, have been determined by two independent procedures, the nebula being first delineated as a bright object on a dark ground, and then as a dark object on a white ground. I cannot enter here into the descriptive details given in the analysis of Prof. Bond's memoir, and I shall confine myself to a report of its conclusion. The general aspect of the greater part of the nebule of Orion is an assemblage of tufts or curvilinear pencils of luminous matter, emanating from bright masses near the trapezium, extending towards the south, on each side of an axis passing by the apex of the region called Huygens, of which the angle of position is in the neighborhood of 180°. Some twenty of these circumvolutions have been distinctly traced, whilst others, producing the same impression, are too faint or too complicated to be described with precision. We may class, then, according to Prof. Bond, the nebulæ of Orion among the spiral nebulæ, such as they were, for the first time, described by Lord Ross, with the aid of his great reflecting telescope. The nebula No. 51, of the catalogue of Messier, was the first in which he discovered this spiral conformation, which had escaped both the astronomers Herschel. Prof. Bond has observed that, in a great number of cases, the masses of nebulous matter are associated with stars, frequently under the form of small tufts extending from their southern side. He cites two remarkable instances where there is a deficit of luminous matter near stars of considerable brilliancy; the first, in reference to the trapezium itself, whose obscure centre has been remarked by sundry observers; the other, to the star lota of Orion. These peculiarities appear to Prof. Bond to be favorable to the supposition of a physical associa tion of the stars with the nebulæ. The existence of an arrangement in a spiral form of the parts which compose it accords with the idea of a stellar constitution; for among the objects which present this peculiarity of form are found not only nebulæ resolvable into stars, but masses of stars properly so called, such, for instance, as the grand mass of stars of the constellation Hercules, where the exterior stars have evidently a curvilinear arrangement. OTHER FACTS RELATING TO THE NEBULE. M. Norman Pogson, whilst at the observatory of Dr. Lee, at Hartwell, in 1860, witnessed a change in the nebulæ, or mass of stars, No. 80 of the catalogue of Messier, situated in the constellation of the Scorpion, and very close to a pair of variable stars R and S of the Scorpion, which have been observed by M. Chacornac since 1853. The 9th of May this nebulæ had its usual aspect, without any stellar appearance, and the 28th of the same month Mr. Pogson saw therein a star of the 7th or 8th magnitude, which has been also observed since the 21st of May by MM. Luther and Auwers at Köningsberg, and which the latter have estimated to be of something more than the 7th magnitude. The 10th of June following, with a magnifying power of 66, the stellar appearance had nearly passed away, but the nebula had a greater brilliancy than usual, with a clearly marked central condensation. M. Pogson does not think that this variation can be attributed to a change in the nebulæ itself, but he regards as singular that a new variable star, the third comprised in the same field of vision, should be found exactly situated between the earth and that nebulæ. This observation has been published in the twenty-first volume of the M. N., p. 32. M. Chacornac has observed quite recently, with M. Foucault's great reflecting telescope of plated glass, so adapted as to procure a great degree of enlarge ment, the annular nebule of the Lyre, and he has ascertained that it is in reality resolvable into a mass of very small stars, closely crowded together, the brightest of them occupying the extremities of the small diameter. This nebulæ, in an examination of several nights, presented to him the appearance of a hollow cylinder, seen in a direction nearly parallel to its axis; and its centre, as Lord Ross describes it, is veiled by a curtain of nebulous matter, which converts itself into a somewhat thin stratum of little stars. M. Chacornac adds, in a communication to Dr. Peters on this subject, dated Paris, 9th June, 1862, and published in No. 1368 of the A. N., that when the view is screened from all interfering light, the scintillation of this multitude of luminous points, occupying a large portion of the surface of the retina, produces a sort of giddiness which is quite curious. This I pass now to the labors of M. d'Arrest, relative to the nebulæ. astronomer had begun to occupy himself with this subject while he was still attached to the observatory of Leipsic, and, since 1857, has published in the collection of the memoirs of the Royal Society of Saxe the result of his first observations of 230 nebulæ, made with a double annular micrometer, of Frauenhofer's construction, applied to a telescope of 52 lines opening and 6 feet focal length. Prof. d'Arrest is at present director of the observatory of Copenhagen, and he has continued, since the month of September, 1861, his observations of the nebulae, with a large achromatic telescope, of 11 inches opening and 16 feet focal length, the optic power of which he estimates to be intermediate between that of Herschel's 20 feet reflecting telescope, and that of the telescope of the same kind with which Lassell likewise has observed the nebula from 1852 to 1854. The telescope of Copenhagen has enabled M. d'Arrest not only to recognize all the nebula of Herschel, but to discover more than a hundred new ones among 776 observed in eight months. He has been enabled also, under favorable circumstances and with some difficulty, to see certain nebulæ indicated by Lassell. M. d'Arrest, making his observations alone, soon perceived that he could scarcely combine the observation of celestial objects of very feeble light with the microscopic reading of the circles of his instrument. It follows that his new catalogue will not assign, with all the precision attainable, the absolute position of each object on the celestial sphere. This position is only given to the minute of a degree in right ascension and in declination; but as the nebula are very carefully compared with the neighboring small stars by the help of annular and thread micrometers, we shall thus have competent means for ascertaining with precision their proper movements in respect to those stars, which constitutes one of the principal aims of the researches of M. d'Arrest. This astronomer has published, in No. 1366 of the A. N., an interesting notice, dated 20th May, 1862, of his later labors; and from this I shall extract some details, tending to complete those which precede. VARIABILITY OF THE BRIGHTNESS OF THE NEBULE. M. d'Arrest considers as well established one of the results of the great labor of Argelander, in which has originated his new catalogue of stars, namely, that, of 50.000 stars already well recognized, there exists but a small number of which the brightness is periodically variable; and he believes the same may be affirmed, though with less certainty, to be very nearly the case with the nebulæ. Sir W. Herschel had subdivided the nebula into three classes, with reference to their relative degree of brightness. M. d'Arrest has found a great many instances in which the nebulæ, as at first classed by Herschel, must now be assigned by one or even two units a new place in the classification. Herschel himself had, in the course of some years, changed several of his appreciations. But in view of the great diversity of atmospheric influences in humid climates, bearing upon observations of this kind, M. d'Arrest thinks, like M. Otto Struve, that it is impossible to be too circumspect in regard to the conclusions to be deduced from variabilities of this nature. He instances, however, a small number of cases in which some degree of variability has been positively ascertained. The first of these cases is that resulting from the observations of M. O. Struve on the nebula of Orion before spoken of. The observations of this nebula recently made, at different times and in favorable nights, by M. d'Arrest, with his large telescope, have confirmed those of M. Struve, particularly as regards * See M. N., vol. xvii, p. 48. the bridge over the great strait, which, last winter, was sometimes distinctly visible at Copenhagen, presenting the appearance assigned it by M. Lassell. The second case of well-established variability is the almost total disappearance of a small and faint nebulæ discovered by M. Hind, October 11, 1852, in the constellation Taurus, recognized by other astronomers, and in the beginning of 1856 still readily perceptible with a telescope of six feet focal length. Two years later it was no longer to be seen, except with great difficulty, in the heliometer of the observatory of Königsberg. It was invisible October 3, 1861, with the great telescope of Copenhagen. M. Chacornac, with the new telescope of M. Foucault, and M. Lassell, at Malta, with his reflecting telescope of four feet diameter, sought for it in vain in 1862, though it was still to be seen with the great achromatic telescope of Poulkova. A curious circumstance, connected with the great diminution of the brightness of this nebula, is that this diminution has coincided with that of a small star which presented itself almost in contact with the nebula. M. Argelander estimated the magnitude of this star, in 1852, at 9.4. It was of not more than the tenth magnitude in 1858, of the eleventh in 1861, and of the thirteenth or fourteenth in 1862. Sir John Herschel believed that he had recently detected another instance of the disappearance of a nebula from not having found inscribed in the first catalogue of M. d'Arrest a very faint nebula, noticed by Sir W. Herschel, near two others in the constellation of Berenice's hair. But M. Chacornac has ascer tained, with the new telescope of M. Foucault, that this feeble nebula is still plainly visible, and M. d'Arrest has also observed it with his great telescope. The latter astronomer further cites a small number of cases where there may have been variability of brightness, or even disappearance of nebula; but these instances are not as well established as that of the nebula of M. Hind. DOUBLE NEBULE. Sir John Herschel has remarked in his important memoir on the nebulæ, published in the Philosophical Transactions for 1833, p. 502, that the number of nebulæ physically connected with one another is probably more considerable, relatively to the total number of the nebulæ, than is that of double stars among the fixed stars.* Admitting a mutual distance of five minutes of a degree to be the greatest for the double nebulæ, M. d'Arrest even now enumerates about fifty comprehended within this limit, and is of opinion that there may be two or three hundred of them among the whole number of some 3,000 nebulæ discernible in our heavens. † So considerable a proportion of double nebulæ justifies the presumption that there is a real connexion in these groups, and their aspect confirms this idea, particularly in the case where unusual forms present themselves at once in two equal exemplifications. Sir W. Herschel seems not to have had an idea of this physical connexion between the nebulæ, but Sir John distinctly speaks of it on more than one occasion. It can scarcely be doubted that at some future period astronomers will be called on to calculate the orbits of the double nebulæ. M. d'Arrest mentions some particular cases of this sort of nebulæ, of which one is triple. There is as yet but one recognized, where, on comparing the distances and respective positions of the two nebula of the same group observed in 1785, 1827, and 1862, considerable changes have been noticed, which seem to indicate a movement of revolution of the one around the other. This double A short analysis of this admirable paper of Sir J. Herschel, accompanied with a plate, is given in the issues of the Bibliotheque Universelle for June and July, 1834. + M. d'Arrest has quite recently published, in No. 1369 of the A. N., a catalogue, for the commencement of 1861, of the positions and aspect of fifty double nebula which he has already recognized, and of which a dozen are new ones. and particularly interesting nebula is situated at 109° 12' of right ascension, and 29° 45′ of north declination. M. Lassell has represented it in Fig. 9 of Plate XI, accompanying his memoir, published in vol. xxiii of the quarto collection of the Astronomical Society of London. The two components of the nebula are very distinct, though their mutual distance is at present but 28 seconds; but they are difficult to be seen when the threads of the micrometer are illuminated. A very small star is found between the two, exactly in the same place where M. Lassell observed it ten years ago. M. d'Arrest will take occasion, when his labors on this subject are completed, to cite some other analogous cases of change of relative position in double nebulæ. His presumption, in the mean time, from what he has been able thus far to discern, is, that there will not be found in any of these groups of nebulæ as short periods of revolution as those which have been verified in the case of some of the double stars. Finally, M. d'Arrest reports a small number of cases where, by comparing a nebula with some small star near it, and repeating this comparison at the end of a certain time, he has been able to verify slight differences of distance or of position, which might indicate a proper movement in one or the other of those bodies. I here terminate this short review, in which I have been able to give but a rapid glance at the present state of observation in respect to one of the most difficult and least advanced parts of astronomical science.* Post scriptum.-M. d'Arrest has just announced, in No. 1378 of the A. N., that he has recognized in the constellation Taurus the existence of a second nebula of variable brightness. *I ought here to correct an error, pointed out to me by Dr. Hirsch, which I committed in my notice on the observatory of Neufchatel, inserted in the number of the Archives for last July, volume xiv, p. 224. It is not M. Hirsch, but Professor Kopp, of Neufchatel, who forms part of the meteorological commission instituted by the Helvetic Society of Natural Science. 20 s |