Chapter XXXIV: Planetary Nebulæ

Planetary nebulæ seem to be intermediate between nebulous stars and annular nebulæ. Indefinite aureolas are replaced in them, as if through the spreading outward of nebulous matter towards a limiting spherical surface, by compact discs, and with the further advance of this process of exterior condensation, the discs become rings. A stellar nucleus persists throughout these phases. They may not be strictly phases of development. To establish an actual sequence of growth, facts of various orders should be considered. As a simple matter of fact, however, objects are found which combine so closely the visual features of planetary nebulæ and nebulous stars on the one side, and of planetary and annular nebulæ on the other, that their classification in the above order is prescribed inevitably, if only for mental convenience.

As a preliminary to the physical study of these objects, we must try to attain a clear conception of their real forms in solid space. This is not easy when their structural complexities are taken into account; but, setting these for the moment aside, we can gather some indications regarding the general plan of their fabrication. As a rule, planetary nebulæ are markedly elliptical. They may then be concluded to be spheroidal in shape; and even those sensibly circular are probably spheroids viewed along their shorter axes. That their compression is due to axial rotation is a fair inference, verifiable, possibly, by spectroscopic measurements. These were indeed ineffectually tried by Professor Keeler, 3rd April 1891, on a bright planetary in Hydra (N.G.C. 3242);[970] but he doubted whether the conditions of the experiment permitted the detection of a difference in the velocities of the advancing and retreating limbs of the nebula of less than seven or eight miles a second, and the movement of such bodies is likely to be excessively slow. Otherwise they could scarcely be supposed capable of holding together; for they are obviously of tenuous composition, and gravity at their equators can act very feebly in equilibrating centrifugal impulse.[971] And here an untrodden field opens to enterprising inquirers. A theoretical investigation might, to begin with, be attempted of the figures which should belong to rotating globes of the kind on certain probable assumptions as to their nature and modes of movement; and the results might be tested by the application to a number of promising objects of Keeler’s spectrographic method, in which the linear images of an equatorial slit serve, through the tilt imparted to them by the contrary motions of the opposite limbs, to measure, in miles per second, the speed of gyration. Small, lucid, strongly elliptical planetaries would be the most hopeful subjects for experiment; nor would a negative upshot be without value. The Hydra nebula, for instance, which is nearly as much flattened as the globe of Saturn, is equal in light to a seventh-magnitude star; it is about 20″ across, and somewhat diffuse at the edges.[972] With it may be compared a condensed planetary in Ophiuchus (N.G.C. 6572), sometimes called Struve No. 6 (Σ 6). Its light, which is comparatively intense, emanates from a small disc 8″ in diameter (exclusive of a hazy margin), with an ellipticity of about ¹⁄₂₀₀. Professor Keeler’s drawing of the object with the visible part of its spectrum is copied in Plate XXIII. Fig. 2. The nucleus is not stellar, although it gives strongly continuous light. The lines in its spectrum were found by Keeler to be displaced upward by an amount corresponding to a motion towards the earth of 6·3 miles a second;[973] but this motion should be considered to belong to the sun, not to the nebula. Judging by its appearance, axial acceleration may have advanced farther in it than in most members of the class, and might record itself in an equatorial spectrograph. The trial, at any rate, should be made. Anticipation of failure need not be allowed to paralyse effort. In science the rush of a forlorn hope often carries a fortress that has obstinately held out against a siege in regular form.

The uniformity of aspect at first supposed to characterise planetary nebulæ disappears before the searching scrutiny of the powerful telescopes now in use. Their surfaces prove to be full of suggestive detail. They are broken up by irregular condensations, or furrowed by the operation of antagonistic forces; they betray, here possibly the action of repulsive, there of attractive influences, and bear inscriptions of no less profound historical import than the contortions and faultings of terrestrial strata. They are quite commonly multiplex formations. One glimmering disc is superposed upon another, intimating the analogy of the successive filmy envelopes flung round the heads of active comets. With the twenty-seven-inch Vienna refractor Vogel succeeded, in 1883, in resolving the above-mentioned planetary in Ophiuchus (N.G.C. 6572) into three distinct layers,[974] doubtless representing concentric ellipsoids of unequal antiquity. A triple outpouring of matter at age-long intervals seems recorded. And the case is typical, although the nebulous shells are not often so clearly defined. This, however, is, on the one hand, a matter of telescopic seeing; on the other, perhaps of evolutionary progress. By way of illustration, let us take as the first term of a sequence an average nebulous star, such as that detected by Auwers in Auriga (N.G.C. 2175); next in order we might place an object near 16 Cygni (N.G.C. 6826), in which the “glow” has become compacted into a large, round, seemingly uniform disc.[975] Sir William Herschel described it as “a beautiful phenomenon, of a middle species between the planetary nebulæ and the nebulous stars.”[976] The following term of the series may be found in a planetary not far from γ Eridani (N.G.C. 1535), consisting of an eleventh-magnitude star, embossed upon two concentric nebulous shields; or rather, presumably, enclosed within a pair of nebulous globes. If we suppose these to have been produced by successive outflows, checked at a limiting surface, we could easily see that the occurrence of a third access of ejective energy would complete the model of the triple specimen in Ophiuchus. It may be added that the difficulty appears almost insuperable of explaining the growth of multifold planetaries on the hypothesis of simple contraction. A repulsive agency in a manner asserts its past activity.

Let us now imagine one of these spherical envelopes to be hollow. The effect to the eye would be that of a luminous ring. And, in fact, an appreciable proportion of planetary nebulæ show an interior circlet of dim radiance, which can hardly be otherwise interpreted than as the projection on a plane of a vast nebulous bubble. This mode of construction is fully carried out in “annular nebulæ”; it is partially realised in not a few specimens in which a ring within a disc is perceptible or conspicuous. All will be considered together in the next chapter; in this we are concerned only with examples devoid of—so to speak—symptoms of inflation.

PLATE XXIV.

1. Photograph of the Owl Nebula (Roberts).
2. Photograph of the Orion Trapezium (W. H. Pickering).
]

The largest planetary in the heavens lies south-east of the second Pointer, β Ursæ Majoris. Discovered by Méchain in 1781, it was numbered 97 on Messier’s list (N.G.C. 3587), and has been extensively and carefully observed. Its dimensions were given by Lord Rosse in 1874 as 163″ by 147″;[977] but the major axis measures 203″ on a photograph taken by Dr. Roberts, with an exposure of four hours, 20th April 1895,[978] and reproduced, by his kind permission, in Plate XXIV. Fig. 1. The observational history of the object is extremely curious. Sir John Herschel saw “a large, uniform, nebulous disc, quite round, very bright, not sharply defined, but yet very suddenly fading away to darkness.”[979] On 2nd March 1848, however, Lord Rosse perceived a doubly perforated surface, with a star in each cavity,[980] and his observations were confirmed by Dr. Robinson. The drawing representative of them is a record of permanent interest. The resemblance to an owl’s face, given to it by the symmetrically placed _oculi_, is unmistakable; and the great planetary in Ursa Major became known, from the middle of the last century, as the “Owl Nebula.” The name has survived the similarity. On 9th March 1850 the two stars were noted at Parsonstown shining as usual in their respective excavations; five weeks later the fainter one had vanished;[981] nor could it ever again be found, though looked for about forty times during the ensuing quarter of a century. To Professor Keeler in 1891[982] the nebula wore indeed an entirely different aspect from that previously attributed to it. “There is but one nucleus,” he stated, “which is by estimation almost exactly central, at a place which in Lord Rosse’s drawing is occupied by a bridge of light between two dark openings. There is also but one central dark space.” Nevertheless, two appear in the Crowborough picture, like lagoons separated by an isthmus, and on the isthmus there is planted, as it were, a lighthouse, diffusing a brilliant illumination. Thus the interior vacancies remain in _statu quo_, while the stars that formerly occupied them have both faded out of sight, leaving the prodominance to a third, not identical with either. The change, in short—admitting that there has been change—relates, not to the structure of the nebula itself, but to the relative brightness of three connected stars. The question whether those at present extinct will ever become revivified, can be answered only by prolonged experience. Dr. Roberts thought that his photograph indicated for the nebula a combined ring and disc formation;[983] but this is not manifest. The picture affords no verification of the marginal inequalities recorded at Parsonstown; yet there is reason to believe that they were not illusory. The torn and jagged contour, which Professor Alexander expounded as the effect of “disruption and dispersion outward,”[984] may come into view in representations on a larger scale, taken with special precautions for the definition of minute details.

The suspected alteration of the Owl planetary accentuates the need for keeping watch over nuclear stars, more especially since they present unexplained peculiarities. Their light is of purely stellar quality, but of remarkable actinic power. They are very much brighter chemically than visually. Further, they often give nebulous images on the sensitive plate, while appearing sharp with the telescope. In some rare cases (as in N.G.C. 6781), they are eccentrically situated. Burnham is probably justified in regarding the possession of a central star as an essential feature of planetary nebulæ,[985] although it is occasionally undiscernible with the telescope. N.G.C. 6563 in Sagittarius, and N.G.C. 7354 in Cepheus, are examples of nebulous discs unrelieved to the eye by the sparkle of any stellar points; and a beautiful little planetary in Perseus, discovered by Barnard 11th December 1890,[986] is of similar aspect. The missing nuclei, however, of all such objects would, it is tolerably certain, become manifest in long-exposed photographs, which should accordingly be taken for the purpose of deciding a point of fundamental importance in the economy of planetary nebulæ.

Perhaps the most noted member of the class, from the numerous experiments of which it has been made the subject, is located in Draco (N.G.C. 6543), quite close to the pole of the ecliptic. To ordinary observation it presents a greenish-blue surface, 22″ by 18″, centred on a vividly white star of the tenth magnitude; but Professors Holden and Schaeberle recognised, with the Lick refractor, its “helical” conformation.[987] Two brighter intersecting hoops, perceived at a glance to diversify the disc, fell into position on closer scrutiny as the thread of a screw, uniting their curves into one continuous tri-dimensional spiral. The discernment of this novel form—believed to be typical—was a suggestive contribution towards what might be termed the solid geometry of nebulæ. Speculations as to its mode of origin would, however, be premature until, by the unerring testimony of the camera, it has been definitively proved to subsist.

The Draco planetary was observed on the meridian by Lalande 26th July 1790, and there are no good grounds for holding it to have since shifted appreciably from the place then assigned to it. Burnham, it is true, has found a progressive diminution of the distance between the central star of the nebula and one external to it, amounting to 0·033″ annually;[988] but the movement, if real, may confidently be ascribed to the disconnected star. The nebula is indeed very far from being stationary in space. Professor Keeler determined for it spectroscopically in 1891 a velocity of approach towards the solar system of forty miles a second;[989] and its apparent fixity on the sphere is doubtless only an effect of extreme remoteness. Some attempts to determine its annual parallax[990] have resulted only in showing it to be small “below compute.” From the prismatic examination of this object, 1st August 1864, Sir William Huggins learned the existence of gaseous nebulæ.

A nebula in Cygnus (N.G.C. 6826) was described by Mr. Burnham as “almost an exact duplicate” of the planetary in Draco. It is nearly circular, the longer diameter measuring nearly 27″, the shorter 24″. The nucleus is very bright (8·8 magnitude), and gives a strong continuous spectrum. The light from the disc, too, includes an unusually large _white_ ingredient.[991] The radial motion of this nebula is very small; it approaches the sun by 3·3 miles a second. But the sun is travelling on its own account towards its place at a much higher rate of speed, so that the apparent sluggishness of the nebula indicates that it is really moving away from us, its measured pace representing only the velocity with which our system gains upon it.

The spectra of planetary nebulæ are fundamentally alike; they differ only in details. About forty bright lines have been determined in them, visually and photographically, and they are invariably fine and sharp, as if emitted by materials of great tenuity. They seem hazy only just where they cross certain diffuse nuclei; and this feature seems to imply a gradual condensation of the nebulous stuff towards the central mass, which, in such cases, cannot properly be called a star. The essential characteristic, however, of nebular light is the presence in it of the ray, or rays, of “nebulium.” This is the one sure criterion by which gaseous nebulæ can be distinguished from stars.[992] The chief nebulium line has a wave-length of λ 5007, and is of a clear green colour. With it is constantly associated a ray about one-third as bright at λ 4959, and the invariability of their relation lends strong probability to the opinion that both emanate from the same substance.[993] A strong ultra-violet line at λ 3727, photographed for the first time by Sir William and Lady Huggins in 1882, perhaps claims an identical origin. This, however, remains doubtful in the absence of decisive evidence that it is an unfailing constituent of the nebular spectrum. The green gas designated “nebulium” is unknown, so far, terrestrially; nor has it been observed to shine in any of the heavenly bodies except nebulæ. There is reason to believe it denser, or at any rate less diffusive than hydrogen. An object catalogued in the Southern Durchmusterung as a ninth-magnitude star under the title S.D.M. −12° 1172, was found by Mrs. Fleming in 1891 to give the spectrum of a planetary nebula. But hydrogen glows in it with unusual intensity. The relative brightness of the three green lines is estimated by Campbell[994] to be ordinarily 10 : 3 : 1, F (Ηβ) being the faintest and most refrangible. But the proportion in the planetary near Rigel is 10 : 3 : 7. In other words, hydrogen is of seven times its normal lustre comparatively to nebulium. Further, the three lines, when viewed through an open slit, form discs of severally 11″, 9″, and 14″ diameter. Here then, apparently, a hydrogen-envelope constitutes an outer shell to the nebulium-sphere; and a gas that rises higher than another is presumably specifically lighter, although, in view of the enormous altitudes attained by calcium vapour near the sun, the inference must be regarded as subject to qualification. Certainty on the point, and on many others connected with the physics and chemistry of nebulæ, may be said to be unattainable until nebulium is captured in the laboratory. And the prospect of this achievement, although not hopeless, is remote.

The following table gives the wave-lengths and origins, when they are known or can be conjectured, of forty bright lines in the spectra of planetary nebulæ. A few are common to all, notably the trio in the green, with about half a dozen of the blue and ultra-blue hydrogen lines, while others are more individual in their occurrence; but, on the whole, bodies of this class seem to be of remarkably uniform constitution.

LINES OBSERVED IN THE SPECTRA OF
PLANETARY NEBULÆ.

Wave-Length. Origin.
6563 Hydrogen (Hα).
5876 Helium (D_{3}).
5751 Oxygen?
5680 Unknown.
5540 Unknown.
5412 Hydrogen; Pickering series?
5313 Unknown.
5183 Unknown.
5007 Nebulium.
4959 Nebulium?
4861 Hydrogen (Hβ).
4790 Unknown.
4743 Unknown.
4715 Unknown.
4688 Hydrogen; Rydberg series?
4662 Unknown.
4643 Nitrogen?
4610 Nitrogen?
4597 Nitrogen?
4574 Silicon?
4472 Helium.
4390 Helium.
4363[995] Unknown.
4341 Hydrogen (Hγ).
4265 Unknown.
4145 Unknown.
4122 Helium?
4102 Hydrogen (Hδ).
4067 Unknown.
4026 Helium.
3970 Hydrogen (Hε).
3968[995] Unknown.
3889 Hydrogen (Hζ).
3869[995] Unknown.
3836 Hydrogen (Hη).
3795 Hydrogen (Hθ).
3768 Hydrogen (Hι).
3727 Unknown.
3460 Unknown.
3390 Unknown.

Several of the Wolf-Rayet lines, it will be observed, are comprised in this list, notably those at λ 541, λ 469, and λ 464. And the absence of recognisable metallic rays strengthens the analogy with stars of that peculiar description. As a rule, the hydrogen spectrum in planetaries begins with the green line. The red line has been distinguished in only a few specimens, which appear more condensed than the rest. Yet it would be rash to assume that this is really the case.[996] The relative intensity of the hydrogen lines in stars and nebulæ is an intricate subject, the ramifications of which have yet to be tracked out. The coincident appearance of D_{3} with C is worth notice as a hint that the conditions favourable to the development of the slower light-vibrations are the same for helium as for hydrogen. They are markedly present in Struve’s planetary in Ophiuchus (N.G.C. 6572 = G.C. 4390), which gives a complex spectrum of at least thirty lines,[997] accompanied by faint continuous radiance. Three classes of fact regarding it are recorded in Professor Keeler’s sketch of the portion of it accessible to eye-observations (see Plate XXIII. Fig. 2): first, the positions of the component rays in the scale of wave-lengths; next, their relative lustre; thirdly, the extent of the nebula from which they are derived. And it is of interest to perceive that only those associated with nebulium, together with the green and blue lines of hydrogen (Hβ and Ηγ), seem to reach the limits of the disc, while the others are radiated only by its central parts. Still we have to remember that the length of the lines must depend to some extent upon their intensity; and that they may be short only because the sections of them given out by dim regions of the nebula are of evanescent faintness. Professor Keeler suspected the presence of dark bands interrupting its continuous light between D_{3} and λ 5007,[998] and they will, if verified, supply the only extant proof of absorption in gaseous nebulæ. The nucleus of Σ 6 apparently reinforces the emissions from the disc; but it is plainly not a genuine star. Further spectrographic investigation of the object is most desirable.

In the Draco planetary (N.G.C. 6543), on the contrary, the stellar and nebular elements of the spectrum are perfectly distinct. Some of the bright lines can indeed be seen only when the central star is outside the slit, and are therefore due, in Professor Campbell’s words, “to the nebula proper, as indeed are all the lines observed, and there is no evidence to show that they exist at all in the central star.”[999] A few of those registered in Struve’s planetary are missing here, particularly the hydrogen line (Pickering series) at λ 541; but C, D_{3}, and both the Wolf-Rayet blue radiations are perceptible, while the leading ultra-violet line at λ 3727 appeared conspicuously in Von Gothard’s photographs.[1000] The relative strength of the green lines in this nebula, as determined by Campbell, is 10 : 3 : 2. It is of a verd-antique hue, and, indeed, the nearly total suppression of red rays in their light gives to all planetaries a blue or greenish tinge.

One of the Durchmusterung stars in Cygnus (D.M. + 41° 4004) was noticed by the late Prebendary Webb, 14th November 1879, to have a hazy disc, some 10″ in diameter.[1001] Stephan, at Marseilles, independently detected its nebular character, and Winnecke compared it to a small comet with a tenth-magnitude nucleus at its preceding end. Moreover, the nucleus is double. Professor Keeler described the object (N.G.C. 7027) as follows:[1002]—“This is the brightest nebula that I have examined, and its spectrum is exceedingly interesting. The nebula is irregular in outline, and contains two central condensations, one of which has an oval and fairly well-defined outline. The other is much fainter and more diffuse.” His drawing of it, made at the great telescope, is copied in Plate XXIII. Fig. 3, together with a representation of its spectrum. The nuclei are obviously non-stellar. The continuous spectrum of even the more conspicuous member of the pair is not incomparably brighter than that derived from the disc, and it claims all the emission rays as properly, though not exclusively belonging to it. Professor Keeler accordingly regarded it as “in a much less condensed state than the nuclei of Σ 6 and many other nebulæ of its kind,” its exceptional brilliancy notwithstanding. The spectrum of Webb’s planetary is remarkable for the intensity of some usually quite subordinate lines, especially of the Rydberg hydrogen ray (λ 4688), and of the unknown lines at λ 4743 and λ 4363. Campbell could just identify the red glint of C,[1003] which by its faintness evaded Keeler’s survey; D_{3} was made out by both observers, and an unidentified line at λ 3869 was photographed by Von Gothard in 1892, although his plates were blank at the place where the significant λ 3727 was expected to appear.[1004]

A companion to Webb’s nebula, both in physical aspect and by vicinity in the sky (N.G.C. 7026), was discovered spectroscopically by Dr. Copeland in 1880.[1005] It had, however, been observed telescopically by Mr. Burnham seven years earlier.[1006] It is small, bright, duplex, a pair of diffuse nuclei 6″ apart sustaining a filmy structure which, viewed with the Lick refractor, suggested a comparison to two sheaves of corn laid side by side.[1007] Burnham refuses to admit the planetary nature of either of the objects in Cygnus,[1008] yet their spectra scarcely allow them to be relegated to a different class. The “Rydberg line” is equally prominent, relatively to their brightness, in both objects. From the cosmogonic point of view they are of high illustrative importance. We seem to have before our eyes double stars in slow course of formation, and preparing to break loose by the development of systemic revolutions from the trammels of a joint rotation.

Bi-nuclear planetaries are not uncommon. One such (N.G.C. 3195) was observed by Sir John Herschel in the south polar constellation of the Chameleon, and is depicted in his volume of _Cape Results_. The nuclei are fairly well matched in lustre, and will perhaps grow into a double star like γ Virginis. Their spectrum is still unrecorded; but it will be of considerable interest to determine whether it exhibits the peculiarities visible in those of the double planetaries in Cygnus. Two bright patches near opposite margins of the circumference give the nebula in Chameleon somewhat the air of a reduced copy of the celebrated “Dumb-Bell” in Vulpecula.

Of a planetary in the Poop of Argo, originally discovered by the elder Herschel, Lassell wrote at Malta about 1851:[1009]—“No description can do justice to this singular object,” which is “not beautiful, for it has no symmetry, but wonderful.” His drawing shows the disc, which had appeared to d’Arrest perfectly round, as pear-shaped, with multiple condensations. So that a quadruple or quintuple star in embryo may here be offered for our contemplation.

“Stellar” nebulæ have been mostly discovered by Pickering’s method of sweeping with a direct-vision spectroscope. There is probably no radical difference between them and planetaries, for their comparative minuteness may be a simple effect of distance. Or they may be constructed on a reduced scale. We should naturally expect to meet in nebulæ a variety of dimensions not inferior to that existing among stars. Moreover, they are all alike gaseous, and give—so far as is yet known—perfectly similar spectra. Nevertheless, Mr. Burnham writes of the stellar kind as “very small, bright, round nebulæ, which in a small instrument would resemble stars slightly out of focus, but do not appear to come within the planetary class.”[1010] An admirable specimen was detected spectroscopically by Pickering 16th July 1882. Previously registered as a 9·4 magnitude star, it took rank, on the strength of its bright lines, as a stellar nebula (N.G.C. 6790). The Lick thirty-six-inch showed it to be round and lucent, with a minute nuclear point.[1011] A miniature of Struve’s planetary in Ophiuchus seemed to float in the field of the telescope. Without a slit, the spectrum, examined by Keeler, resembled three tiny greenish _stars_, that formed on Hβ being much the faintest. This stellar nebula accordingly is analogous to Webb’s planetary in the feeble glow of its hydrogen constituent. Another pseudo-star in Aquila (N.G.C. 6891) disclosed itself prismatically to Dr. Copeland in 1884. It has a disc just 4″ across, and its spectrum, photographed by Von Gothard 27th October 1892,[1012] includes the usual range of nebular lines up to λ 3727, besides a fair admixture of continuous light. A nebula of the fourteenth magnitude, visually a finished star, was noticed for the quality of its light by Pickering, 25th November 1881, near the star _b_^2 Cygni.[1013] More conspicuous members of the class have been identified in considerable numbers on the Draper Memorial plates.

The crowd of small nebulæ photographed by Dr. Max Wolf in 1901 comprised a remarkable proportion of seeming planetaries. They were collected on his plates into pairs and groups in a manner recalling the distribution of Wolf-Rayet stars, but not previously observed to characterise that of planetary nebulæ. The question indeed arises whether they are really such? Or do they rather belong to “that much less interesting class of objects” designated by Mr. Burnham as “small circular patches of nebulosity”? The spectroscope alone can decide, and its verdict should be elicited without delay. Upon it will largely depend the conclusions to be drawn respecting the affinities of the planetary family, their mutual relations, and the mode of their scattering in space.

There is reason to believe them enormously remote. Four have been directly measured for parallax, namely, the helical nebula in Draco, the bi-nuclear planetary in Argo (N.G.C. 2440),[1014] Webb’s in Cygnus, and a structure with a ring and disc in Andromeda (N.G.C. 7663).[1015] None showed the least sign of perspective shifting when viewed from opposite sides of the earth’s orbit; and the demonstration thus afforded of their immense distance is confirmed by the insensibility of nebular proper motion. This, since it is continually progressive, must eventually prove determinable, and comparisons of its amount with the mean _radial_ velocity of these bodies will supply a criterion for their absolute localisation. Professor Keeler derived from the line-displacements in the spectra of eleven nebulæ a value for this quantity of sixteen miles a second; and by sixteen miles a second they should, accordingly, on an average, progress along each of the other co-ordinates fixing their position in space. When corresponding angular advances have been established, their average distance can then at once be estimated.[1016] Let us assume, for instance, that the eleven nebulæ in question, taken one with the others, have a secular proper motion of three seconds of arc in declination, and as much in right ascension. This, although the outside of what is probable, implies that their mean distance corresponds to a light-journey of 580 years. A planetary not more than 10″ in diameter would, if thus remote, fill a globe about 600 times wider in girth than one circled by the orbit of Neptune, and 216 million times more capacious. The data supplied by Professor Keeler are indeed an obviously insufficient groundwork for extensive generalisations; but more of the same kind, and it may be hoped of not inferior quality, cannot fail to be forthcoming shortly; while the precise visual measurements executed by Burnham, Spitaler, Javelle, and others will surely serve, after some decades, for the detection of genuine nebular journeyings across the sky. A beginning of definite knowledge will then have been made regarding the true magnitudes and place in the sidereal scheme of these singular objects. It must not, however, be forgotten that many of the faint stars taken as fiducial points for their micrometrical determination may prove to be satellites of the neighbouring gaseous globes drifting and shifting in their company. This disquieting possibility, foreseen by Sir John Herschel and d’Arrest, would, of course, if realised, vitiate conclusions as to parallax or proper motion. Yet the work done for these purposes need not be looked upon as wasted. By its aid, should the slow revolutionary movements of stars round planetary nebulæ ultimately be brought to light, a new department of sidereal mechanics may be founded. Exact determinations in astronomy, made on a judicious plan, are rarely useless. If unprofitable for their designed aim, they are pretty sure to turn to account for some other, and perhaps a higher one.