A STAR ATLAS KOK THE LIBRARY, THE SCHOOL, AND THE OBSERVATORY, 6,000 STARS AND 1,500 OBJECTS OF INTEREST, IN TWELVE CIRCULAR MAPS ON THE EQUIDISTANT PROJECTION ; TWO COLOUKED INDEX PLATES, IN THEIR PROPER RELATIVE POSITIONS, INCLUDING ALL THE STARS TO THE FIFTH MAGNITUDE, AND THE CONSTELLATION FIGURES. Bv RICHARD A. PROCTOR, B.A. (CAMBRIDGE), FORMERLY HONORARY SECRETARY OF THE ASTRONOMICAL SOCIETY, AI-THOI! ciK -'SATl'KX AND ITS SYSTEM," "OTHER WORLDS THAN OURS," "SUN-VIEWS OK TUB EAHT1I,' ETC. KTO. WITH A LETTER-PRESS INTRODUCTION. THIRD EDITION. Kr (I T(i rcifxa TROTH, TUT' u'vuiirw; lim^afwrni. HOVER. luvra "EitiV/r TTuXi I'wpu' TO. yap Kat TTUITU /^fi\' avru ' ayaX/iara rvKroc louarie. ARATU.S. LONDON : LONGMANS, GREEN, & CO. 1874. LI J5 It A it V I' N I V KI.'S I')' V ()!' CALIFORNIA. T71 LONDON \VVMAN AND SONS, PKINTKliS, GHKAT " from the centre of projection, is as difficult as to present a map extending round the centre on all sides to that distance. Such a map would cover exactly a third part of the sphere. Obviously no plane chart can include such a large proportion of a globe as this without great distortion. Again, to present a map having angles removed 5444'8 - 2" from the centre, would be equivalent to presenting a circular map having an arc-radius of this magnitude. Such a map would cover considerably more than a fifth part of the sphere f ; and though this is * There is another mode of conceiving the division of the sphere into equal and similar figures which is worth noticing. Suppose the centre of a sphere to be a luminous point, and that a regular solid lies within the sphere, all its angles lying on the surface of the sphere. Then if the edges of this solid are opaque, the solid itself being transparent, their sluidows on the sphere's surface will be the bounding lines of equal and similar spherical divisions. t The actual extent of such a map would, of course, bear to the area of the sphere the ratio which the versed siim of r,\~- 44' 8-2" bears to 2 : this is easily found to be about the ratio of 21,137 to 100,000. In the case of the do.!. .-;,. liedmn, the corresponding proportion of the sphere is represented by the ratio 10,267 to 100,000, OY less than half the former. But a more truthful, and even more effective, way of comparing the two modes of division is tlie following : Of all central projections, the equidistant is the one which least markedly displays the defects of maps covering INTRODUCTION. 5 better than the former case, yet it is still wholly impossible to present so large a proportion of the sphere without great distortion. Now, when we consider the case of a map having angles removed 37 22' 38'5" from the centre, we find a great diminution in the extent of surface we have (in effect) to deal with. For a circular map having an arc-radius of this magnitude covers very little more than a tenth part of the sphere (in reality '10267 of the sphere). Even this, however, might at first sight seem more than we can present -without con- siderable distortion ; and it is in considering the method of resolving the difficulty that all other modes of projection but the one here made use of are eliminated. Only three central projections are applicable to such a problem, the gnomonic, the stereographic, and thr> equidistant. The gnomonic method is used in the index-maps, a glance at which will show that, however useful the projection may be for special purposes, it is wholly unequal to answer our chief purpose, which is to get rid of all marked distortion. It is clear that the spaces between meridians and parallels are markedly different near the angles of the index-maps than on the sphere, and that test is decisive. The stereographic projection seems more promising. It is a peculiarity of this mode of projection that it exhibits no distortion whatever so far as small figures are concerned, and also that all angles remain unchanged. Now, if this absence of distortion extended to large figures also, the stereographic projection would have so great a superiority over all others in this respect that we could hardly conceive that superiority in other respects could bring other plans on a level with it. But of course this cannot be the case. It is obvious that a mode of projection which exhibited small and large parts of the sphere undistorted, would exhibit all parts on their true scale, and this no plane chart can possibly do. It remains then to consider whether the stereographic or equidistant projection gives the most favourable result, when scale-variation, area-variation, distortion of small areas, and dis- tortion of large areas, are all considered together, and due weight given to the advantages of either projection in each several respect. In the stereographic projection applied to a map having an arc-radius of 37 22' 38'5', the maximum scale-variation is from 1 to I'll 6, while in the equidistant projection the scale- variation is but from 1 to T075; so that while the scale increases by more than one-ninth in the stereographic projection, it increases by less than one-thirteenth in the equidistant. The area-variation is from 1 to 1'245 in the stereographic, and from 1 to 1'075 in the equidistant ; so that while the area-scale increases by nearly one-fourth in the stereographic projection, it increases by less than one-thirteenth in the equidistant. Now as regards distortion, it will be obvious that the advantage derived from the absence of distortion of small areas in the stereographic projection is fully balanced by their greater increase of size as compared with what appears in the equidistant projection ; for in judging of a small group of stars the eye will be quite as much deceived by a modification of its size as compared with other groups, as it would be by a slight modification of its figure. But the distortion of small areas in the equidistant projection, altogether insignificant and to most eyes inappreciable in the present instance (as any one will see by comparing the spaces between meridians and parallels in the centre of any of the maps in this Atlas, with the corresponding spaces near the edge of the map), is far more than counterbalanced by the superiority this mode of projection has over the a large part of the sphere. Therefore this method will give the most favourable results for the cubical division of the sphere. Now the distortion and scale variation on this projection are measured by the ratio which the excess of arc over sine of angular range of a map from the centre bears to the sine of this angular range. Thus we have the following proportion : f distortion in cubical ) f distortion in dode- ) .. arc 54 44' sin 54 44' . arc 37 23' sin 37 23' I division of sphere ) \ cahedral division / sin 54 44' sin 37 23' 13886 4532 :: : or about as 2i to 1. 81647 60714 And in this ratio, at the very least, does the dodecahedral mode surpass the cubical mode of division. C 6 INTRODUCTION. stereographic as repects the small distortion of large areas. It need hardly be said that if one part of a constellation is affected as respects area by an increase of nearly one-fourth, the whole aspect of the constellation cannot but be appreciably altered ; which is not the case where no part of a constellation is affected by an increase of so much as one-thirteenth. In addition to these conclusive considerations, the equidistant projection will give smaller maps than the stereographic, on a given scale. Thus the equidistant projection is to be preferred to the stereographic in this case. And further, no one can doubt that while the stereographic projection introduces a change of area sufficing to render that projection unsuitable for a star-atlas, the equidistant projection, which gives no greater distortion than that resulting from a longitudinal increase of parts of the maps by less than one-thirteenth of their extension on the sphere, does really meet the requirements of the case. In other words, it has been proved that the plan of projection used for the first time in this work is the only one which is fit to be applied to the construction of a celestial Atlas.* Already, too, it is manifest that no Atlas hitherto constructed could give clear ideas respecting the laws according to which the lucid stars are distributed over the heavens. For, in order to discover such laws, a uniform mode of dividing the sphere is absolutely essential ; and the only uniform mode of division actually adopted until now, is that employed in the maps published by the Society for the Diffusion of Useful Knowledge. Now, in these maps a jnode of projection is made use of which actually gives an increase of scale from 1 to 5'19 ; so that in certain regions of the heavens stars might be spread in reality five times as ricldij as elsewhere, and yet in these maps those very regions might be presented as relatively some^vhat barer than the rest.f The only plan really fit for constructing a Star- Atlas having been discovered, it remained to consider how that plan might most effectually be applied. Remembering that the heavens are not marked with meridians and parallels, or with circles of longitude and latitude, but with stars, it would be for many reasons convenient if maps could be arranged with reference to the actual configuration of star-groups rather than to the imaginary lines and circles used by astronomers. Two men of science, both of great eminence in their respective walks, have indeed suggested to me the adoption of some such plan. But when I considered how closely the accepted methods of marking the celestial sphere have become associated with all the processes employed in astronomical observation, I felt that these methods must be employed in any celestial Atlas meant for general use ; and it became further clear to me that if the circles and parallels of declination are to be introduced into any Atlas, they must be introduced on a systematic and clearly intelligible plan. The connection between the several maps, and their relation to the poles of the heavens and to the celestial equator, must be made clearly recognizable. This admitted, it was obvious that one of the twelve pentagonal maps must have the north pole for its centre, the opposite one the south pole. Accordingly, the first and twelfth maps of the present series are polar ones. The remaining ten thus become equatorial maps, five being northern, the other five southern. Next the meridian corresponding to O h of right ascension must fall either on the middle or along the side of a pentagonal map (two boundaries of each pentagonal map except the polar ones being necessarily meridians). It was a matter of no moment, so far as symmetry f The considerations leading to the selection of the present mode of dividing the sphere and projecting each map are so obvious and simple, that I could not persuade myself for a long time that they had not occurred to astronomers before. Even now, though I have searched in vain for traces of maps constructed on my plan, I should luirdly be surprised if I found that I have unwittingly adopted a plan already advocated l>y others. t This is no imaginary case. A part of C'ygnus, singularly rich in stars, happening to fall near an angle of the cul>e on which, in the Society's maps, the stars are supposed to be projected, appears considerably less rich than regions falling near the centre of some of the maps, where yet there are not in reality half as many stars, taking space for space on the heavens. INTRODUCTION. 7 was concerned, whether this meridian passed through the middle of a northern equatorial inn]), and along an edge of a southern one, or nice versa.* But as one arrangement might suit the actual configuration of the star-groups better, I inquired carefully which was to be preferred. I found that the constellations Orion and Ursa Major would be crossed by the boundary-lines of maps whichever plan was adopted, but that they might be preserved unbroken by an arrangement presently to be considered, if the meridian marking O h -R.A. crossed the middle of a northern equatorial map, whereas Orion at any rate must be divided if this meridian traversed the side of a northern map. This settled the question ; but it remains to be added that the resulting plan happens very fortunately to carry the edges of the maps most conveniently clear of all the principal star-groups, at least when the subsidiary contrivance now to be described is adopted. The index-plates exhibit the real boundaries of the twelve pentagonal maps (except that along the equator the maps are extended beyond their true pentagon, -whose outline is indicated by broken lines). It will be clear that, if these five pentagons were laid down on separate sheets, there would be a difficulty in carrying on the investigation of a star-group, when there was occasion to pass from one group to a neighbouring map. This is a difficulty which all who have used Star-Atlases must often have felt. Now in the index-plates the difficulty is in great part removed, because (i.) the five equatorial maps are brought into actual juxtaposition with their respective polar maps ; (ii.) other edges (ten pairs) are brought near enough for comparison ; and (iii.) overlapping pieces are appended to the remaining ten edges. But, besides that on the scale necessary for such an Atlas as the present, no such plan was available, the mode of projection actually adopted the equidistant does not give straight edges to the pentagonal maps. The idea, therefore, suggested itself to me, as tending to an obvious improvement of the Atlas, that, instead of letting each map include only a spherical pentagon, it should include the whole of that part of the sphere which lies within the circle circumscribing the pentagon. Thus each map overlaps the five neighbouring maps, and a star-group which has been followed in any one map to the edge can be taken up again, not at the edge of another, but well within its limits. Only at twenty points, instead of along thirty arcs each nearly twenty-one degrees in length, could any star-group be inconveniently broken ; and it happens that no angle of any of the pentagonal maps falls where such breaking off could be mischievous. Further, even near these angles three maps overlap, so that were there any star actually upon an angle, it would be brought into association with three differing areas lying all around it, and overlapping each other. It will be seen that the plan of overlapping has saved several important star-groups from being divided. Here and there the boundary-line has been broken to save other groups from division ; and it will be found, on a careful study of the whole series of maps, that all the chief constellations, or rather all the recognized star- groups, are fully presented in one or another map. Orion is given in full (as regards the stars which form its characteristic features) in two maps. The seven bright stars of Ursa Major are given together in one map, and appear divided in two others ; so that the association between this well-known group and three great stellar regions (three-tenths of the whole heavens) can be very clearly recognized. The use of the index-plates in still further indicating the association between the different maps of the series will be also at once obvious. The question of scale had next to be considered. It was necessary, on the one hand, that the scale should be sufficiently large to admit all the stars in the British Association Catalogue down to the sixth magnitude, inclusive, without crowding ; while, on the other, it was desirable that the maps should be of a convenient size. It seemed to me that the scale of an 18-inch globe would meet both these requirements. If the stars which belong * The study of the two index-plates, as compared one with the other, will exhibit the nature of the problems actually involved. INTRODUCTION. to the orders to be included were spread with tolerable uniformity over the heavens, the scale of a 15-inch globe would be sufficient; but as a matter of fact stars are so thickly- congregated in certain regions that a larger scale is required. The scale of the index-maps being that of a 6-inch globe, while only 1,500 stars from the British Association Catalogue fall within those maps, it is clear that a scale giving four times as great an area per star would equally well suit the requirements of maps to include 6,000 stars. Thus the scale of a 12-inch globe would have been sufficient had the index-plates been on an adequate scale for general and observational purposes. But this seemed to me not to be the case. The scale of an 18-inch globe gives more than twice as much space per star as is given by the index-maps. The actual scale of the maps is, however, somewhat larger. The originals having been drawn on the exact scale of a 30-inch globe, it seemed that the best plan in the photolithographs would be to reduce by about one-third, as that would give maps conveniently proportioned to the size of the paper proposed to be used. Thus the scale of the present maps is, as nearly as possible, that of a 20-inch globe, or more than ten times the superficial scale of the index-plates. Next as to the mode of construction. In the polar maps, of course, the equidistant projection simply gives equidistant con- centric circles for the declination-parallels, and a series of lines radiating symmetrically from the centre for the meridians. But in the equatorial maps, neither meridians nor parallels fall into the form of any curves ordinarily constructed. It seemed to me so important that these maps should be actually on the equidistant projection, and not merely approximations, that I determined to calculate the proper place (on any chart) of the intersections of meridians and parallels to every fifth degree, and to take these curves through the points thus laid down, not in any case using any of the appliances for tracing ordinary curves, even when the actual curves closely approximated to circular or elliptical arcs. It seemed preferable that here and there some irregularity of appearance should perhaps manifest itself, rather than that the scheme of projection should be appreciably interfered with. The Atlas shows : All the stars from the B. A. Catalogue to the sixth magnitude inclusive. First, all the nebulas in Sir J. Herschel's Catalogue down to the order marked Very Bright, and all Messier's nebulae, whether so marked or not. Secondly, all the binaries and suspected binaries in Mr. Brothers' Catalogue. Thirdly, all the objects in Admiral Smyth's Bedford Catalogue. Fourthly, all the red stars in Dr. Schjellerup's Catalogue of 293 such stars, using Mr. Lynn's reduced places for the year 1870, as presented in Mr. Chambers' s Astronomy. Fifthly, all the variables in a complete list of all stars actually recognized as variable (136 in all), kindly prepared for me by Mr. Baxendell. In the case of southern maps I have marked as double, triple, &c., all the stars already marked in, which are described as of such a nature in Sir J. Herschel's South Cape Observa- tions, adding a few objects from that work ; not so many as I otherwise should, because the southern maps are more crowded with stars than the northern, and would not readily bear such additions. I have also marked as double, &c., all objects so noted in the S.D.U.K. maps which did not happen to have been included in any of the above-mentioned lists. The constellation outlines follow the simple plan adopted in the Catalogue of the British Association. A few words of comment are required respecting the nomenclature of the stars. All stars having Greek letters are so marked, nor have I thought it advisable to give to such stars, in addition, the number assigned them by Flamsteed, as is done in some atlases. The use of both letter and number, in such instances, not only encumbers an atlas needlessly, but serves more often to cause mistakes than to assist the student. To all stars not having Greek letters, but numbered by Flamsteed, 1 have given his number. To all stars marked with Italic letters, or small Roman capitals, in the four subdivisions of Argo, their proper INTRODUCTION. 9 letters have been assigned in this Atlas. So far as the 5,932 stars belonging to the British Association Catalogue are concerned, these are all the modes of nomenclature systematically adopted. Thus a large number of these stars (especially in the southern maps) remain wholly unnamed. It has seemed to me at least as proper that these stars should be recognizable by their right ascensions and decimations, as compared with those given in tint British Association Catalogue, as that they should be indicated by numbers of three or four figures, referring to the catalogues of Lacaille, Brisbane, and others. But, besides, the main purpose of this Atlas to wit, the truthful representation of the heavens would have been defeated if such numbers as these had been suffered to encumber the maps. As the Arabic names of the stars are still recognized, it seemed advisable that, though all but a few have been banished from my maps, this work should provide the means of ascertaining the Arabic names of a far larger number. An alphabetical list of the constellations is given, with the number of the map in which the constellation is to be found ; and in all cases where they occur on two or more maps, the references outside the borders will be found sufficien.. All objects double stars, nebulas, variable stars, &c. have been numbered or lettered in these maps, where they have received any title. The most convenient appellation has been used where a star has more than one number or letter. A red, double, or variable star which has no recognized number or letter, is marked Eu instead of R., Du instead of D., VA instead of V. The mode of indicating star-magnitudes is new and altogether better, I think, than any before adopted. An important improvement, I think, consists in the adoption of Sir John Herschel's photometric scale of star-magnitudes as a guide in determining the scale of the disc of stars belonging to the three first orders of magnitude. The magnitudes hitherto given in maps are far from representing the true magnitudes of the stars ; and, even did they do so, no star-maps can ever fairly represent the heavens, if no means are adopted for exhibiting the difference between the brightest and faintest stars of at least the leading magnitudes. Owing to the adoption of this plan, the present maps present the familiar star-groups in a manner which cannot but be far more satisfactory to those who study the heavens than the caricatures hitherto presented in our atlases. I should like to see the constellation-figures wholly banished from Star- Atlases ; but as these figures are too preposterous to be readily forsaken, I have so far yielded to the feeling in their favour, as to let them appear in the index-plates. The plan I have adopted for indicating the names of constellations is a manifest improvement on the one usually employed. The name of the constellation is read more easily, and at the same time (which is of far more importance) the star-groups are not disfigured by the presence of a number of letters straggling amongst them. And here I approach with fear and trembling the subject of constellation nomenclature. It has been the recognized practice of those who have drawn new Star- Atlases, to earn a cheap immortality by adding new constellations to the heavens. According as these constellations are more or less preposterous, and as their names are more or less unwieldy, they appear to have had a greater or less chance of becoming fixtures in our atlases. Monoceros, Canes Venatici, and Cameleopardalis attest how clearly Hevelius recognized this great principle ; while Bode has been even more strikingly successful in encumbering the heavens with such noteworthy additions as Honores Frederici, Globum Aerostaticum, and Machina Pneumatica. Again, the multitude of additions made by Lacaille to the southern heavens, Telescopium, Microscopium, Horologium, and whatever else is least celestial and most sesquipedal if they roused a word of protest from practical Francis Baily, yet at once found a host of defenders among Continental astronomers. Therefore, if I had desired that my name should appear in the astronomical treatises of 2870 or 3870, I had nothing to do but to form some such constellations as Tormentu'm D 10 INTRODUCTION. llflUcum Wldtwortliiense, or Spectroscopium Automaticum Br owning ense, in the perfect assurance (derived from the long experience of astronomers) that these new constellations would be joyfully welcomed. Instead of this I have risked an immortality of objurgation by venturing on a small measure of reform. I have endeavoured to diminish the burden of names with which our maps have been encumbered. Let me at once exhibit the full extent of my misdoing. I have altered Triangulum Boreale . . . into Triangula. Triangulum Australe ,, Triangulum. Canes Venatici . . . . ,, Catuli. Corona Borealis .... ,, Corona. Corona Australis .... ,, Corolla. Piscis Australis .... ,, Piscis. Cameleopardalis . . . . ,, Camelus. Vulpecula et Anser . . . ,, Vulpes. Equuleus ..... ,, Equus. Delphinus . . . . . ,, Delphin. Ursa Major ..... Ursa. Ursa Minor . . . . . ,, Minor. Canis Major ..... Canis. Canis Minor ..... ,, Felis. Leo Minor . . . . . ,, Leasna. Monoceros ..... ,, Cervus. In all 93 letters have been struck off these fifteen names, or 192 letters reduced to 99. I thought of doing away with Sagitta, altering Capricornus into Caper, and mulcting Sagittarius of a few letters, but my heart failed me. The Milky Way has been carefully copied from Sir John Herschel's picture, so far as the southern heavens are concerned. Elsewhere I have followed his description as closely as possible, here and there taking details from the best maps, and in some cases referring to the heavens themselves. This last course I found especially necessary in delineating the Milky Way around the gaps in Cygnus; and I was only prevented from applying it more frequently by the consideration that the aspect of the Milky Way must be regarded as a question of eyesight, and others may have seen the details of the galaxy more clearly than I have. The precession-arrows marked in on the latitude-parallels serve to present the effects of precession much more intelligibly than any plan hitherto used. One can at once, by means of these arrows, tell the true place among the stars of any object whose R. A. and dec. (or N. P. D.) are given for any date, even a hundred years before or after the date of the map. Remembering that all the stars are shifting (as respects their R. A. and dec.) parallel to the arrows, and that the nearest arrow to any star presents the star's precessional motion for 100 years (or, where the arrows differ most in length, that the extent of this motion lies between the lengths of the two nearest arrows), we can at once make any desired correction for precession. Thus, suppose we want to mark in an object whose R. A. and dec. are given fpr 1840, we find the stated place in the map, and put the star down in advance of that place, in the direction pointed out by neighbouring arrows, and by a distance equal to the length of two divisions out of five on the nearest arrow. And so all objects given for a date preceding 1880 may be marked in. Objects whose places are assigned for dates after 1880 will have to be shifted backwards. In all cases the change of place is parallel to the neighbouring latitude-parallels. The maps may be regarded as appreciably true for the next thirty years ; and available INTRODUCTION". 11 (owing to the simple way in which the correction for precession is made) for the next century or so. It is to be noticed that, in passing from one map to another, there is no necessity for turning over map after map, as according to every plan hitherto adopted. One has only to look at the number of the map indicated outside the circumference, and to turn to that mnp. Looking round its circumference for the number of the map just left, one finds at once the continuation of the star-group under examination. For example, say one is studying Serpens in the lower left-hand corner of Map 8, and wishes to pass towards 4/ and % Scorpii. Map 9 is written outside the border, and turning to Map 9 one finds Map 8 written outside the upper right-hand border. There one finds the constellation Serpens, and its neighbours Ophiuchus, Scorpio, and Libra, as required. The numbering of the maps is arranged on a very simple plan. The north polar map is numbered 1 ; the map whose central meridian marks Oh. R. A. is numbered 2 ; then the other equatorial maps, alternately south and north, in order of their right ascension ; and, lastly, the south polar map is numbered 12. Every precaution has been taken to make the maps as useful as possible to the observer. Many objects belonging to each of the various classes studied by astronomers have been introduced, while care has been taken to avoid encumbering the map with too many of these objects. Considering that most of these objects are employed (at present) rather as tests for telescopes than for original research, all that might have been deemed absolutely necessary was that enough of them should be marked in to supply the large army of amateur telescopists with that practice by which they seem to be preparing themselves for really useful researches in future years. However, as will be seen by what has been already written, I have gone far beyond this, insomuch that I suppose the total number of objects indicated in the twelve maps cannot fall very far short of two thousand. But there is one purpose hitherto wholly disregarded which a Star-Atlas ought above all things to fulfil. It does not seem to have been hitherto considered that star-maps constructed on an intelligible plan, can supply most useful information respecting the laws of distribution and arrangement which exist among the stars. It is impossible for the unaided eye to recognize these laws even among the lucid stars, for reasons which every one who has studied the heavens will at once recognize. Star-maps, however, if well constructed, can give information as useful in its way as that which even the telescopes can supply. They can render palpable laws which really exist, but escape all other modes of recognition. Now that which has alone rendered the work of star-mapping congenial to my tastes, is the fact that it is in truth a method of research, and a method which is as yet new and untried. I did not complete the two sheets used as index-plates to this Atlas without recognizing the fact that systematic star-charting is a promising method of research ; and I looked forward hopefully to the time when I should be able to extend the method of stars of the sixth magnitude. After four years of unavoidable delay, this became possible ; and the progress of the work has impressed on me most strikingly the value of this mode of research, viz., the systematic charting of the heavens, on an intelligible plan by which the laws of stellar distribution and aggregation may be rendered obvious to the eye. But the present Atlas is only a second step upon this new path. As my new theory of the sidereal system was suggested by the maps here used as index-plates, and many interesting peculiarities of stellar distribution by the larger maps, so I am certain that by extending the process. successively to stars of the seventh, eighth, and ninth magnitude, new facts of unimagined importance would be revealed. I do not hesitate to express my belief that if a hollow globe were constructed on the dark inner surface of which * the 310,000 stars charted by * The stars and other objects could be illuminated on the same principle as the "electric tree," many batteries being employed. The whole construction would be an expensive affair, but the cost would be nothing compared with that of the Kosse reflector. 12 INTRODUCTION. Argelander should be shown, all the nebulae yet discovered, the general results of the Herschel star-gauges, the positions of red, variable, and other remarkable objects, and an accurate presentation of the Milky Way and the Magellanic clouds, a far greater amount of light would be thrown on the relations presented by the sidereal system than could be obtained by the use of a telescope twelve feet in aperture, conveniently mounted, driven accurately by clock-work, and supplied with the finest procurable spectroscopic appliances. For three centuries astronomers have been studying the heavens piecemeal ; it is time that the scattered facts were brought together, and the heavens as interpreted by the telescope exhibited in one grand picture ! The information presented even by these maps, which advance so small a way into the celestial depths, is sufficiently promising to encourage high hopes of what might be done by applying a similar plan of research on a far wider scale. In the first place, let me indicate the singularly disproportionate numbers of stars found in maps which each cover the same proportion of the heavens. Considering only the stars belonging to the British Association Catalogue (the professed object of that catalogue being to include all the stars visible to the naked eye), we have in the six northern and the six southern maps the following numbers : Northern Maps. Southern Maps. 1 contains 693 stars. 12 contains 1,182 stars. 2 ,, 397 ,, 3 ,, 523 ,, 4 526 5 834 _ 6 415 7 547 ' 8 390 9 595 10 563 11 528 Total 2,984 Total 4,159 Excess of stars in southern over stars in northern maps . . . 1,175 Since each map covers one-tenth of the heavens instead of a twelfth, we shall not be far wrong if we take as the numbers falling within the true limits of the northern and southern maps respectively 2,487 and 3,466 stars.* This makes the excess of stars in the southern maps still nearly 1,000. The actual excess of visible stars in the southern hemisphere is rather more than 1,000; and it is only the overlapping of the equator by alternate maps, which reduces the surplus. Are not all the laws of probability against the existence of so remarkable a disproportion, save as the result of special laws of aggregation ? But looking more closely into the details of these numerical relations, we see that three maps stand markedly apart from the rest. First there is the south polar map with its array of 1,132 stars ; then far behind the south polar map, but still far in advance of all others, comes Map 5 with 834 stars ; then follows the north polar map with 693 stars, or 100 beyond the average. Map 9 has about the average number of stars ; and all the remaining stars except three have a number not falling importantly below the average. These three maps (2, 6, and 8) contain between them but 1,202 stars, or but 70 more than the south polar map alone contains.! Now it is only necessary to look at Maps 3 and 5 to see that there is (as we might feel certain beforehand from the nature of these numerical relations) a law of aggregation in the * That this is not far wrong is shown by the fact that the sum of these numbers (5,953) is only 21 beyond the actual number of stars extracted by me from the British Association Catalogue. Of course, the overlapping causes the difference. t The first half of the south polar map contains more than GOO stars, while the first half of Map 8 contains but 150. INTRODUCTION. 1 :; i southern heavens. In these maps we recognize the boundary of a southern region rich in lucid stars. Less clearly, but still unmistakably, the outline of this rich region can be traced across Map 7. In Map 9 the boundary is irregular and straggling, and markedly associated with the peculiarities of the Milky Way. In Map 11 a very small portion of this rich region is seen trenching upon the lower left-hand part of the map. Map 12 is wholly occupied by this great region of stellar aggregation. We notice further that the greater Magellanic cloud occupies the very heart of this region ; while the most cursory study of Map 12 will show how intimately the Magellanic clouds are associated with the branching system of lucid stars extending through, or rather forming, the great southern rich region. It is also obvious, from the arrangement of the stars over the parts of the Milky Way shown in Map 5, Map 12, and Map 7, that large groups of lucid stars are intimately associated with the masses of minute orbs forming the Milky Way here. In the northern heavens one recognizes a faint reflection of these relations. There is a tolerably well-marked, roughly circular, northern region, rich in stars, occupying the lower right-hand part of Map 1, traversing the upper left-hand corner of Map 2 (its boundary exceedingly well defined near Lacerta), and extending itself with straggling boundary over the upper right-hand portion of Map 10. It is noteworthy that a single branch of the Milky Way joins the two regions, on one side, the complex part of the Milky Way joining the regions on the other side. Each of the rich regions includes a well-marked "coal-sack" in the galaxy, and corresponding to that projection which in the southern rich region the galaxy throws out from Carina towards the greater Magellanic cloud, there is in the northern rich region the projection thrown out from Cepheus towards (Ursa) Minor. It is also worthy of notice that the barest regions of the heavens are found on the borders of the rich regions, and along the edges of the Milky Way notably in Cervus (Monoceros), and in that remarkably bare rift extending from Auriga past the feet of Gemini. One can recognize the prolongation of this bare region around the borders of the rich region in Map 1. It may be also remarked, that all the well-marked star streams recognized by the ancients as Eridanus, Hydra, and the streams from the water-can of Aquarius flow into the southern rich region. While drawing these maps, I have been struck by the manner in which red stars and variable stars are associated either with the Milky Way or with star-streams, and also by the frequent occurrence of streams in which are included lucid stars, red stars, variable stars, and conspicuous nebulas. The fact that red stars, variables, and bright nebulas are seldom (if at all) found alone in barren regions of great extent, is very significant ; and by no means opposite in meaning to the fact noticed by Sir W. Herschel, that nebulas begin to be found where star-groups end. One other matter, of several which have suggested themselves while the work has been in progress, seems to merit a word of comment. I think I have detected the plan on which the ancient astronomers associated the constellation-figures with star-groups. By ancient astronomers I refer, of course, to those compared with whom Ptolemy and Hipparchus are moderns. It must have struck every one who has compared star-groups and the figures with which they are associated, that no resemblance, can be recognized save in one or two instances. Now in drawing Map 6, I was very much struck by the striking resemblance between a star- grouping covering a large portion of the map and the figure of a lion. 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