2 74 



NATURE 



[July 21, 1892 



the work itself. There are citations to the number of 

 423, and more, not numbered, yet, although we have a 

 long discussion on the relationship of Limulus to Scorpion, 

 Lankester's work is not mentioned ; with chapters on 

 fresh-water faunas, no allusion to " The Origin of Fresh- 

 water Faunas," by SoUas ; W. Marshall, a German 

 author, is set forward in the text as an authority on 

 pelagic and coast faunas, and Moseley overlooked ; titles 

 are sometimes given without place or date of publication, 

 a defect which becomes serious when periodical literature 

 is referred to without mention of volume. The illustra- 

 tions are numerous and excellent. 



The author has produced a fresh and promising 

 thought, but one cannot help regretting that he did not 

 wait — like, say, Darwin — till it was full time for bringing 

 forth. W. J. SOLLAS. 



THE PHOTOGRAPHIC MAP OF THE 

 HEAVENS} 

 'T^HE first number of the second volume of papers 

 -*• published under the auspices of the Permanent 

 Committee charged with the execution of the photo- 

 graphic map of the sky has made its appearance at a sad 

 moment in the history of the undertaking. For simul- 

 taneously with its appearance is announced the death of 

 him who, more than any other man, has contributed to 

 its success, and brought it within the range of practical 

 science. Admiral Mouchez has known how to secure not 

 only the active co-operation of many astronomers, but 

 also how to make them zealous in the great work, the 

 arrangement of the details of which has occupied the 

 last years of his life. He has awakened enthusiasm for 

 the success of his scheme, and smoothed many difficulties 

 which might have hindered its progress, and probably 

 few undertakings of equal magnitude and equal im- 

 portance, breaking new ground in many directions, have 

 been got under way with less friction and fewer dis- 

 appointments. We may well hope that the same sauvity 

 and diplomacy which has characterized the conduct of the 

 late Director of the Paris Observatory will be found in 

 the counsels of his successor, and that a work begun in so 

 much hope will be carried to a successful issue. 



The papers in the volume before us can be brought 

 roughly under two heads, both, notwithstanding the lapse 

 of time from the inception of the scheme, betokening an 

 initial stage in the preparation. One of the topics under 

 discussion has for its aim the selection of a method which 

 shall secure on the photographic plates, destined ulti- 

 mately to furnish a catalogue, the impression of stars of 

 the eleventh magnitude with certainty and uniformity ; 

 the other, a means of deriving the co-ordinates of the star 

 images so impressed with the greatest facility and 

 sufficient accuracy. 



To deal with the second of these proposals first, we 

 may remind our readers that whatever method of 

 measuring the positions of stars on a plate may be 

 adopted, the resulting co-ordinates must be purely 

 differential, and probably referred to the axes of the 

 reseau impressed upon the plate as a latent image, and 

 developed under the same conditions as the stars them- 

 selves. To pass to the determination of R.A. and declina- 

 tion, a great deal of information, entirely independent of 

 photography, will have to be made available. The readiest 

 means of effecting this last step in the reduction, as it 

 appeared to a committee of experts appointed to consider 

 this question, was to determine by meridian instruments 

 the absolute co-ordinates of six stars on each plate. It 

 is needless to comment upon the magnitude of the labour 

 thus undertaken, or at least contemplated. This pre- 

 liminary work would demand a catalogue of some sixty 

 or seventy thousand stars, most of them below the ninth 



I " Bulletin du Comite International Permanent," tome ii.,' premier 

 fascicule. 



NO. 1186, VOL. 46] 



magnitude and not found in existing catalogues. In 

 order to give to each determination the necessary accuracy, 

 it is desirable that each star should be observed twice in 

 both elements and at two observatories. When we 

 remember the length of time that the re-observation of 

 Argelander's zones has consumed, and is still incomplete, 

 we can form some estimate of the time that must inevit- 

 ably elapse before the results of the photographic catalogue 

 can be made available for astronomical purposes. 



In presence of these difficulties, and many more which 

 occur to the practical astronomer, we must be very grate- 

 ful to M. Loewy for elaborating a scheme which, if it be 

 found practicable, will materially shorten the time neces- 

 sary for the production of the catalogue. M. Loewy 

 proposes to avail himself of the fact that the plates are 

 taken in two series, in such a manner that each corner of 

 a plate in one series will form the centre of four other 

 plates in the second series. When, therefore, the 

 astronomer has determined the rectilinear co-ordinates 

 of the stars on one plate relatively to the central lines of 

 the reseau, each of these stars will belong in common to 

 the plate considered, and to one of the four plates of the 

 second series, partially covering the first. M. Loewy's 

 scheme consists in making the stars on the four plates 

 thus connected available for the reduction of the first. 

 And, on paper at least, it is not difficult to extend the 

 scheme still further, and to make the plates contiguous to 

 these four contribute to the reduction of the original 

 plate by means of an extended triangulation. In this way 

 a plate would not be considered as an isolated fact, but 

 a considerable area, of 36, 64, 100 or more square degrees 

 could be woven into a harmonious scheme of reduction. 

 And such a plan possesses this very obvious advantage, 

 that on even a lesser area, as of 36 square degrees, we 

 may well expect to meet a sufficient number of bright 

 stars whose places are already so well determined that 

 the reduction of the plates could go on immediately with- 

 out waiting for the observations of the stars on the 

 meridian. And independently of this evident advantage, 

 it seems highly probable that two of the elements of 

 reduction, viz. the orientation of the plate, and the value 

 of the scale, will be determined more accurately, if the 

 stars which are used for the derivation of these corrections 

 are separated by a considerable distance, that is greater 

 than a single negative would permit. 



M. Loewy considers the various sources of errors and 

 their necessary correction with all the detail required to 

 submit the plan to practical application, and this is pre- 

 cisely the test that is needed. This appears to be also 

 the opinion of Dr. Gill, expressed in a very cautious 

 approval of M. Loewy's scheme, and he further quotes a 

 remark of Prof. Auwers, which contains a very salutary 

 caution. That astronomer points out that the reduction 

 of the catalogue plates will be most accurately effected 

 from the position of faint stars, rather than from bright 

 ones. In that case since our present most accurate cata- 

 logues do not give the positions of the fainter stars, those 

 catalogues will still need to be supplemented by many 

 meridian observations. Dr. Sande Bakhuyzen, however, 

 expresses the opinion that the zones of the Astronoinische 

 Gesellschaft will, when completed, furnish the necessary 

 data for all reductions, or, at most, require additional 

 observations in some portions of the sky, which he is 

 able to point out from a careful examination of the 

 number of the stars contained in these zones. 



The second topic which has received much considera- 

 tion in this volume is, as before mentioned, the adoption 

 of a method to secure the registration of stars of the 

 eleventh magnitude. It will be remembered that the 

 International Congress of 1891 proposed to place in front 

 of the object glass of the telescope, screens of fine metallic 

 gauze, identical in manufacture, and of such construction 

 that the amount of light impeded should be equivalent to 

 two magnitudes : the coefficient 2-512 being employed as 



