NATURE 



509 



THURSDAY, MARCH 29, 1900. 



CELESTIAL PHOTOMETRY. 

 Photometrie der Gestirne. Von Prof. Dr. G. Miiller, 

 iljservator am Koniglichen astrophysikalischen Ob- 

 rvatorium zu Potsdam. Pp. x + 556. Mit 81 

 • ujuren im Text. (Leipzig : W. Engelmann, 1897.) 



IN addition to the issue from time to time of the 

 results of original research, the staff of the Potsdam 

 Observatory displays its activity and manifests its use- 

 fulness by the publication of text books, remarkable alike 

 for their thoroughness and for the mastery of the subjects 

 of which they treat. We recently called attention to 

 Prof. Scheiner's work on celestial photography, and we 

 have now to notice a book published about the same time, 

 by his colleague. Dr. Miiller, on photometry, a subject 

 which he has made peculiarly his own, especially in re- 

 searches permitting the application of the ZoUner form 

 of photometer. But if in practice he has limited himself 

 fb the use of this particular apparatus, his familiarity 

 with the history and literature of the subject, his know- 

 ledge of the use and construction of other forms, and his 

 intimate acquaintance with all that has been accomplished 

 in this comparatively recent branch of astrophysics, mark 

 him out as the proper authority for the production of a 

 book on photometry, and constitute him a safe guide and 

 able instructor in an inquiry that is not without its 

 difficulties. 



Dr. Miiller divides his book into three main sections ; 

 the first treats of the elementary principles on which 

 photometry is based, the second deals with photometric 

 apparatus and methods, while the third is devoted to the 

 detailed consideration of the results of photometry ap- 

 plied to celestial objects. In the first section, various 

 elementary problems are discussed, which have been 

 treated by numerous authors, from Lambert downwards. 

 Over these one can rapidly pass, since they occupy, to 

 some extent, a ground common to all photometric 

 inquiries, and have extensive application outside astro- 

 nomical research. The so-called Purkinge phenomenon 

 and Fechner's psychophysical law are discussed at 

 sufficient length, and may be found attractive since recent 

 controversy has invested these subjects with considerable 

 interest. Of more direct bearing on celestial photometry 

 are the albedo of the planets, the distribution of light 

 over planetary discs, the effects of phase, &c. The latest 

 views of physicists, naturally of German physicists, on 

 these questions are set out in great detail, and various 

 problems which have been discussed in periodical 

 literature are here collected into a convenient and 

 accurate summary. In the third and concluding chapter 

 of the first section we have presented to us a full dis- 

 cussion of the important, but rather thorny, inquiry into 

 the amount of light lost in passing through our atmo- 

 sphere. The theoretical treatment that this problem has 

 received at the hands of Lambert, Bouguer, Laplace and 

 Maurer (the last better known in this country in con- 

 nection with the names of Dale and Gladstone) is 

 copiously examined, and not only is justice done to the 

 eminent mathematicians by whom the inquiry has been 

 elaborated, but we have also a comparison between the 

 NO. 1587. VOL. 61] 



results of theory and the amount actually deduced from 

 observation by empirical methods, oV, at least, by methods 

 in which no theoretical assumptions are made. On the 

 side of practical observation, Dr. Miiller has himself 

 worked with vigour and marked success. His table of 

 values, which express the average amount of light ex- 

 tinguished by the atmosphere, as determined for the 

 Potsdam Observatory, probably holds the most authori- 

 tative position of all the inquiries that have been instituted. 

 In saying this, we do not leave out of sight, as Dr. Miiller 

 has apparently done in his table on p. 138, the work of 

 Profs. Pickering and Bailey in this department of photo- 

 metric inquiry. It is difficult to understand the reason 

 for this omission. Prof. Pickering's method of observing 

 the brilliancy of a circumpolar star at its upper and lower 

 culmination seems to be free from objection, and has the 

 certain advantage of confining the observations to the 

 plane of the meridian, the plane in which all the ob- 

 servations are made. The amount of his material was so 

 large that some roughness and inaccuracy in his ob- 

 servations would disappear in the results, and his simple 

 formula of -025 mag. sect. Z, applied to the observed 

 magnitude, appears to us quite as deserving of mention as 

 some of the results here collected. Some exception 

 might be taken to Dr. Miiller's own value given in the 

 table (p. 138). He has himself made two determinations 

 of the value of the coefficient of transmission of light 

 through the atmosphere ; one conducted in the presumably 

 clearer and drier air of the Santis summit, the other at 

 Potsdam, where the results are affected by the dust and 

 vapour due to the proximity to a busy town, but which 

 probably fairly represent the meteorological conditions 

 that prevail in the neighbourhood of most observatories. 

 To the results of the Santis investigation no exception 

 can be taken ; but, notwithstanding the general favour 

 in which the Potsdam table is held and accepted, it has 

 the disadvantage of containing implicitly two values of 

 the transmission coefficient. This curious result is due 

 to a change in the manner of observation when dealing 

 with objects at small altitudes. Practically no difficulty 

 will arise in the use of Dr. Miiller's table of observed 

 extinction, but we decline to follow the author in his 

 assertion that this peculiarity is an advantage. In the 

 first part of the table, the amount of light extinguished 

 indicates that 81 per cent, of the incident light falling on 

 the atmosphere is effective ; while the latter part would 

 show that 85 per cent, emerges. Both of these values 

 cannot be correct ; but the coefficient 0-835, suggested 

 by Dr. Miiller, represents not only the mean of his own 

 observations, but accords very satisfactorily with the 

 average of all the determinations that have been made. 

 From the theoretical side, it is safe to say that the theory 

 of Laplace represents the actual observations within the 

 accuracy at present attainable. 



This agreement between theory and observation leaves 

 untouched Prof. Langley's contention concerning the 

 total loss of radiant energy in a ray of light passing 

 vertically through the atmosphere. While there is no 

 question as to the legitimacy of the objection that Prof. 

 Langley has raised. Dr. Miiller is not inclined to accept 

 the high percentage of loss demanded by the American 

 physicist. Dr. Seeliger is quoted as an authority of 

 equal weight, to show that the probable loss of light is 



Z 



