15G 



KNOWLEDGE. 



[Ji-LY 1, 1895. 



been made. The labours of Dr. Weinek, referred to in the 

 June number of Knowledge, may be cited as a case in 

 point. The original photographs have been taken at the 

 Lick Observatory, or by MM. Loewy and Puiseux at the 

 Paris Observatory," and Dr. Weinek has reproduced them 

 on a greatly enlarged scale, which has brought into 

 prominence many new and minute details. Again, at the 

 Cape Observatory, a work of the first order of importance 

 has been carried out— a complete photographic survey has 

 been made of the southern heavens. The measurement 

 and reduction of this valuable series of plates was impossible 

 at the Cape Observatory itself, owing to the smallness of 

 the staif and the total insufficiency of the funds placed by 

 the Government at the disposal of the Director. But what 

 the parsimony of the British Clovernment failed to do has 

 been accomplished by the devotion of a foreign astronomer, 

 and the plates of the Cape Ihii-chniustcrunij have been 

 measured and reduced by Prof. Kapteyn. 



This work is one not of merely national, but of inter- 

 national importance, and it is not to be expected that 

 similar tasks can often be undertaken, or that any amateurs 

 could in the general way take part in them. An example 

 of a useful piece of work within the power of many 

 amateurs may be seen in the reduction, by Mr. A. 

 Stanley Williams, of a series of photographs of Jupiter 

 taken and measured at the Lick Observatory, and com- 

 municated by him to the Royal Astronomical Society 

 (Monthly Notices, Vol. LI., p. 402). 



But that amateurs may undertake work of this descrip- 

 tion, it is first of all necessary that they be supplied with 

 the photographs. This was the first object Mr. Ranyard 

 had in view in the reproduction of so many photographs 

 in Knowledge. This, too, was Dr. Roberts' chief 

 purpose in the publication of his volume of " Photo- 

 graphs of Stars, Star-Clusters, and Nebulfe." These 

 photographs have been carefully reproduced in a form to 

 render them available for measurement and reduction. 

 The scale value is given for each plate, and four or five 

 fiducial stars are indicated on each, and their places for 

 the epoch 1900 given, so that the positions of other stars 

 may be readily ascertained. 



These particulars will be carefully given by Dr. Roberts 

 for the photographs which he proposes to publish in 

 Knowledge. They will not, therefore, be meielij beautiful 

 pictures — that they will be such those who know 

 Dr. Roberts' published volume will feel assured— they will 

 be available for strictly scientific treatment, and it is to 

 be trusted that they will not fail to receive it. 



The first use to which these photographs may be 

 expected to be put is to determine the relationship between 

 stellar distribution and nebular structure. For this 

 purpose the published paper reproductions will not be 

 greatly inferior to the original negatives, and it is to be 

 hoped that there are not a few careful observers who will 

 subject them to the most detailed scrutiny. What would 

 Michell, what would the Herschels, what would Proctor, 

 have given to have such records placed in their hands ? — 

 records which, in Dr. Roberts' own words, " portray 

 portions of the starry heavens in a form at all times 

 available for study, and identically as they appear to an 

 observer aided by a powerful telescope and clear sky for 

 observing." 



* By a careless slip, the pbotogi-aph from which the enlargements 

 reproduced in the June number of Knowiedge were u;ade by 

 l)r, Weinek was said to bare been taken by the Brothers Henry 

 (p. 13.5, col. 1, lino 34). 'the instrument with which the photo- 

 graph was obtained— tlip Equatorial Coude — was indeed due to 

 their skill, but the actual photogr.ph was taken by MM. Loewy and 

 Puisettx. 



THE GREAT NUBECULA. 



By E. Walter Maunder, F.R.A.S. 



THE readers of Knowledge are indebted, as on so 

 many previous occasions, to the skill and success 

 of the Director of the Sydney Observatory, Mr.H. C. 

 Russell, for the photographs which are reproduced 

 in the present number. Like the pair of photo- 

 graphs which we published in April, these were taken with 

 the Sydney " Star Camera," i.e., with the telescope con- 

 structed for the International Astrographic Survey, and of 

 the standard aperture and focal length. They represent 

 two contiguous fields in the very heart of the Greater 

 Magellanic Cloud, the northern edge of Plate I. correspond- 

 ing to the southern edge of Plate II., and they should be 

 compared with the fine photograph of the same region 

 taken by Mr. Russell with a si>:-inch portrait lens on 

 October 17th, 1890, and published in Kno\^-ledge for 

 March, 1891. The earlier photograph received an exposure 

 of 7h. .Sm., and is remarkable for the vast masses of nebu- 

 losity which it revealed ; nebulosity the spiral character 

 of which was distinctly shown upon the original plate, 

 constituting it, as Mr. Russell remarked, "the grandest 

 spiral structure in the heavens." The present pair of 

 photographs are much less impressive regarded merely as 

 pictures, for they bring out much less extended nebulosity. 

 This is the necessary consequence of the shorter exposures 

 given to them— the exposure for No. I. being 4^ hours and 

 for No. II. 5j hours — and of the greater proportional focal 

 length of the star camera as compared with the portrait 

 lens. On the other hand, the numbers of the stars shown 

 are far greater on the present plates than on that taken in 

 October, 1890, and the scale is larger, so that minuter 

 details are seen. The earlier photograph was taken on a 

 scale of 0'55() inches to the degree; the accompanying 

 plates are on a scale six times as great, being enlarged 

 from the originals, which gave a scale of 2-3(5 inches to 

 the degree. 



There is, perhaps, no object in the heavens which has 

 been of such importance in the development of our views as 

 to cosmical structure as the Great Nubecula. For this vast 

 object, extending, according to Sir John Herschel, over forty- 

 two square degrees, is not only the very hive and home of 

 nebulfE — the region of the entire heavens where they crowd 

 most closely — but it is full of groups and streams of im- 

 doubted stars. This one object, therefore, was itself 

 sufficient to disprove the old idea that iiebulffi were but 

 extremely distant galaxies, nebulous only by their untold 

 distance ; for it is plain that the depth of the Nubecula 

 can be only a fraction of its distance from us. However 

 vast, therefore, its real dimensions, we may regard all its 

 members as practically at the same distance from us ; we 

 cannot suppose that its furthest extensions are double as 

 far or even one-quarter as far again as its nearest. It is 

 not due, therefore, to diflerences of distance that it appears 

 to consist, according to Sir John Herschel's well-known 

 description, " partly of large tracts and ill-defined patches 

 of irresolvable nebula, and of nebulosity in every stage of 

 resolution, up to perfectly resolved stars like the Milky 

 Way, as also of regular and irregular nebulas properly 

 so called, of globular clusters in every stage of resolvability, 

 and of clustering groups sufficiently insulated and con- 

 densed to come under the designation of clusters of stars." 

 The diflerences in appearance are due to real differences of 

 aggregation and of condition, and are not simply due to 

 the disguising efl'ects of distance. This lesson, that 

 nebulas were not " external galaxies " irresolvable by eflect 

 of distance, was the first lesson taught by the Great 

 Nubecula. Its second lesson, that nebulae equally with 



