NATURE 



349 



THURSDAY, AUGUST 29, 1872 



SCH ELLEN'S SPECl^KUM ANALYSIS 



Spedntui Analysis in its Application to Terrestrial Sub- 

 stances, and the Physical Constitution of the Heavenly 

 Bodies. By Dr. H. Schellen, Director der Rcalschule, 

 I.O. Cologne. Translated from the second enlarged 

 and revised German edition by Jane and Caroline 

 Lassell, edited, with notes, by W. Huggins, LL.D., 

 D.C.L., F.R.S. (Longman and Co., 1872.) 



IT is not difficult to deliver interesting lectures or to 

 write an instructive book on spectrum analysis. Tlie 

 rapid succession of brilliant discoveries in this new 

 branch of science, the amount of fundamental facts 

 added by it to human knowledge, especially in the 

 field of the cosmical world, assure the lecturer or writer 

 appealing to the intelligent but not scientific public of 

 useful and legitimate success. But what is not so easy to do 

 is to interest at the same time the i^ens du monde and 

 scientific men, by offering a selection of the most recent 

 discoveries in a bright and literary form attractive to the 

 former, and yet keeping for the latter the appearance of pre- 

 cision, and exactness of the numerical results. 



All these conditions are very happily filled in " Schellen's 

 Spectrum Analysis," edited by Mr. \V. Huggins from the 

 second German edition. I shall commence by giving a 

 brief account of the chief points of the book. 



The first part, introductory, is occupied by a descrip- 

 tion of the artificial sources of high degrees of heat and 

 light, of which the study is so intimately connected with 

 the chemical and astronomical phenomena embraced 

 in the field of spectrum analysis ; various apparatus, 

 for instance, the gas-burner, the magnesium lamp, 

 the Drummond lime-light, the electric spark of the in- 

 duction coil, the Geissler's tube, and the electric light 

 produced by voltaic batteries are described, and the prac- 

 tic.-ii adjustments are briefly but sufficiently referred to 

 for a good understanding of the subject. 



The second part is devoted to an elementary abstract 

 of the geometrical and mechanical properties of light. 

 The fundamental analogy between light and sound is 

 developed, in order to explain to a reader unlearned in 

 optics how the colour of a ray is the corresponding ele- 

 ment of the pitch of a musical sound, and how it is pos- 

 sible to define a coloured ray by the time of its luminous 

 vibrations. The description of refraction phenomena, espe- 

 cially the paths of rays through a prism, leads naturally to 

 the separating process of the different colours on which 

 speetrun; analysis is founded. 



Some examples of such an analysis of light by means 

 of a prism are given, amongst which we may mention 

 the screen-projections of the spectra of the electric, sun, 

 or magnesium light ; a sufficient number of illustrations 

 enables every one easily to repeat the experiments. 



A considerable number of chapters is devoted to the 

 construction of the simple and compound spectroscope. 

 The chief points of this construction, especially the con- 

 trivances for the simultaneous comparison of two spectra, 

 the determination of the position of lines in the spectrum 



VOL, VI. 



are carefully described. Afterwards a practical account 

 of the methods for exhibiting spectra of terrestrial sub- 

 stances, for instance, metallic salts volatilised in a gas- 

 burner, &c., will certainly interest chemists. 



The beautiful appearance in the spectroscope of heated 

 gases in Geissler's tubes, their bright lines, and also the 

 important question concerning the change of spectra 

 with temperature and pressure, all these subjects are 

 sketched in general outlines ; it is nevertheless to be re- 

 gretted that an account of the beautiful experiments of 

 MVI. Frankland and Lockyer have not a place amongst 

 these descriptions ; the difficult problems raised by these 

 experiments are not completely solved, and we by no 

 means can accept every assertion developed in this inte- 

 resting chapter. 



The curious absorption phenomena to be observed in 

 the spectrum analysis of light whichhas passedthrough cer- 

 tain liquids, especially of organic origin, deserved a pecu- 

 liar notice ; the author has not neglected to describe 

 one of the most remarkable spectra — the absorption-bands 

 of the blood, and to indicate what advantage natural his- 

 tory is able to derive from such observations even on 

 microscopical objects. 



An interesting chapter contains the theoretical and ex- 

 perimental explanation of the reversal of the spectra of 

 gaseous substances. This phenomenon, studied indepen- 

 dently by Foucault and Angstrom, and definitely general- 

 ised by Kirchhoff, is perhaps the chief point of the history 

 of spectrum analysis, and certainly the beginning of its 

 utilisation as a powerful method of investigation. 



The only practical example of reversal given in the 

 book is that of sodium vapour ; but recent experiments have 

 proved that nearly all metallic vapours heated conveniently 

 in the voltaic arc show the reversal of a great number of 

 bright lines into black ones. 



The third part of the book, the most important in ex- 

 tent and results, is devoted to the application of the 

 spectrum analysis to the heavenly bodies. 



The sunlight, according to its brightness and to the 

 peculiarities of its spectrum, is the best and easiest ex- 

 ample to study. The dark lines in infinite number which 

 it shows, called " Frauenhofer lines," from the discoverer, 

 deserve special attention ; therefore the author has illus- 

 trated the description of the sun-spectrum with two sets 

 of maps. The first is a reduction of Kirchhoff's maps 

 engraved on wood, representing in several tints the lines 

 from A io G ; the second series is a reduction to about 

 half size of the admirable normal solar spectrum of 

 Angstrom, in which the Frauenhofer lines from a to 

 HjHj are co-ordinated according to their wave-lengths. 

 The accuracy of these lithographic plates is really wonder- 

 ful ; they will have the great merit of introducing amongst 

 physicists and astronomers the wave-length scale for the 

 designation of lines instead of Kirchhoff's scale, which is 

 an arbitrary one ; and in any case they will facilitate the 

 transformation of the data from one to another. I must 

 add that Angstrom's maps have been introduced into the 

 present edition by the English editor, and that such an 

 addition is certainly one of the greatest attractions of 

 this book for scientific men. 



A good abstract of Kirchhoff's and Angstrom's memoirs 

 on the coincidence of the dark solar lines with the bright 

 lines of metallic vapours leads to the hypothetical con- 



