364 



NATURE 



[February 15, 1900 



was not needed, nor does it imply that Prof. Scheiner's 

 attempt was hasty and ill-advised ; but it does affect the 

 point of view from which the book is to be regarded at 

 the present day. A second edition is needed to bring the 

 matter up to date in all particulars, and this will no 

 doubt be forthcoming ; but in any case the book will 

 stand as a valuable record, supplied by an expert, of the 

 methods in which the problem of photographic produc- 

 tion and measurement was applied before familiarity and 

 experience had shaped the most suitable method of treat- 

 ment. The author is already favourably known as a 

 writer of high-class text-books dealing with practical 

 work in the observatory, and we can have no hesitation 

 in saying that this book will add to his reputation and 

 that of the Potsdam Observatory, whose staff have from 

 time to time issued a welcome series of manuals. 



Dr. Scheiner treats his subject under three divisions. 

 First, the production and utilisation of photographs ; 

 secondly, photographic photometry and the nature of 

 the photographic image ; and thirdly, the history of 

 astronomical photography and its results. This arrange- 

 ment seems satisfactory, and permits the author to group 

 his facts clearly about the main points at issue, but we 

 doubt whether in the future so much importance will 

 be attached to photometrical measurement, as the 

 prominence here given to that subject seems to intimate. 

 There are not wanting signs that we shall be content to 

 guess the magnitude of a star from its appearance on the 

 film, iust as we judge of its brilliancy in the telescope, 

 though there will always remain specialists who will be 

 content to gather their facts much more slowly, and 

 possibly with greater accuracy, by rigorous measurement 

 of the disc. 



In the first part we have some valuable remarks on 

 photographic technique, in which the author s practical 

 knowledge is shown to great advantage. Of the different 

 methods of development to produce definite results, 

 probably we know as much as we ever shall, but it is 

 not easy to convey the necessary information by precept. 

 No ^student would, however, content himself with mere 

 book knowledge, but would have recourse to actual 

 manipulation in the laboratory, and the value of this 

 preliminary chapter would be forced upon his attention. 

 The second chapter contains a discussion of the ordinary 

 forms of object glasses and mirrors suitable to photo- 

 graphic work, and attention is called to the errors that 

 arise, whether from the construction of the optical parts, 

 or from the manner in which the image is received on the 

 sensitised film. The remarks are clear and pertinent ; 

 but if the chapter were to be written anew, it would prob- 

 ably be felt desirable to dwell more on the photographic 

 doublet, and to contrast the amount of its distortion with 

 that of the ordinary object glass. The peculiarities and 

 advantages arising under certain circumstances from 

 greater variation in the focal length to the aperture 

 might have to be considered, and the peculiar forms of 

 ccelostat now in use would demand more attention. 



In the next and most important chapter, on the methods 

 of measurement and reduction, the author has adhered 

 perhaps rather too strictly to the historical than to the 

 practical side of the question. It would have been, we 

 submit, of greater service to the astronomical student to 

 have possessed in the fullest detail that method which 

 NO. 1581, VOL. 61] 



experience has shown to be of the greatest utility, illus- 

 trated by a numerical example, than the reproduction of 

 a variety of processes which have not met with general 

 approval. And since Dr. Scheiner has already published 

 no insignificant portion of the catalogue of that part of 

 the sky allotted to him by the International Committee, 

 his experience would have enabled him to speak with 

 authority on this vexed question of reduction. Or, dis- 

 liking this method of selection, he might have worked 

 out an example by the different methods, and thus 

 furnished us with the means of exercising our judgment 

 on this important question. The historical student may 

 hereafter be very grateful to him for collecting the 

 methods, which were suggested in the early days, before 

 the photographs existed in any considerable numbers 

 and ingenuity could run riot, untrammelled with the 

 weight of the heavy numerical calculations that must 

 come after. He may choose to weigh and contrast the 

 methods of Bakhuyzen and Jacobi for rectangular, with 

 that of Gill, arranged for polar, co-ordinates. He may 

 linger over the ingenious device of M. Loewy, or take 

 refuge in the more practical method due to Prof Turner ; 

 but in any case he would have been more grateful to the 

 author if he had introduced a uniform notation, and so 

 made the different methods more easily comparable. By 

 simply reproducing the methods as they are given in the 

 original memoirs. Dr. Scheiner has missed an excellent 

 opportunity for rendering an essential service to the 

 cause of clearness and order. This section is apparently 

 complete up to the time of compilation, but various im- 

 provements have been suggested since, which could not 

 possibly find a place here. We are probably far from 

 hearing the last word on this subject of reduction, and 

 possibly still further from the adoption of any method 

 that will commend itself to all the participants in the 

 international scheme, and ensure, or at least attempt, a 

 uniform standard of accuracy. In the last chapter of 

 the first section the author deals with the automatic 

 registration of star transits and of latitude determinations 

 by means of photography. These attempts are admit- 

 tedly in an experimental stage, and some of the instru- 

 ments by which it s proposed to compass the desired 

 end still exist only on paper. The Observatory of George- 

 town appears to be leading the way, but we have no 

 figures by which we can judge of the measure of success 

 that has attended the application. Prof. Turner's pro- 

 posed form of photographic transit is apparently of too 

 recent a date to obtain a critical notice ; the same remark 

 applies to Sir R. Ball's telescope at Cambridge. 



On the subject of photographic photometry, which 

 forms the second section of the book, the author is quite 

 on his own ground, and, writing with authority, he gives 

 us the most complete exposition of the methods which 

 has yet appeared. It is a subject which early attracted 

 attention, when photography first attacked the question 

 of astronomy of position, since many of the decisions of 

 the International Committee were founded upon more or 

 less imperfect knowledge of the growth of the image, and 

 the relations between the time of exposure and the 

 magnitude of a star photographed in a given time. 

 Much information, not all of which was consistent, 

 was rapidly accumulated, and the author deals with this 

 mass of material very satisfactorily. Some of the formula? 



