NATURE 



509 



THURSDAY, SEPTEMBER 20, 1906 



.1 TEXT-BOOK OF OPTICS. 

 I'liYsica} Optics. By R. W. Wood. Pp. xiii + 546. 

 (London: Macmil'lan and Co., Ltd., igo6.) Price 

 15.9. net. 

 A XT' HEN a book on optics by Prof. Wood was 

 VV announced, students expected something 

 interesting, and they have not been disappointed. 

 In his preface the author explains that when lie 

 commenced his work Preston's "Theory of Light" 

 was the only advanced English text-book of modern 

 date available. Since that time Schuster's " Theory 

 of Optics " and the English translation of the late 

 Prof. Drude's " Lchrbuch der Optik " have appeared, 

 and Prof. Wood had to consider whether they covered 

 till' field sufficiently. 



Hi^ readers have cause to be glad that he answered 

 this inquiry in the negative, nor will they regret 

 the fact that he has laid special stress on the experi- 

 mental side of the question, and has devoted consider- 

 able space to the account of some of his own work. 

 His apologies for this are unnecessary; many of the 

 experiments so described are beautiful, and students 

 learn more from reading a man's account of his own 

 work than in other ways. 



Wliilo ihe book hardly claims, perhaps, to be a 

 complete treatise, it covers a great deal of ground, and 

 in particular deals with a number of matters, such 

 .'IS the laws of radiation, dispersion, fluorescence, and 

 the optics of moving media, which are not so fully 

 treated in some other recent works. A student com- 

 mencing the study of optics would perhaps hardly 

 begin with this book; he would find, however, in its 

 pages when he came to read them some most in- 

 structive views of the subject. The earlier chapters 

 deal with the rectilinear propagation of light and its 

 reflection and refraction. They gain much by the 

 photographic reproductions showing the passage of a 

 w.ive of sound through an aperture, and after reflec- 

 tion and refraction at a plane surface, and teachers 

 will do well to insist on the utilitv of the graphic 

 method of studying the changes in wave form, which 

 Prof. Wood uses freely. 



The theoretical treatment of the matter is perhaps 

 less satisfactory ; it is based essentially on that of 

 Verdet. Schuster's paper on the method of analysing 

 in an elementary manner the propagation of a plane 

 wave might with advantage have been alluded to. 

 In dealing with both reflection and refraction, 

 Fermat's law of minimum time is explained at an 

 early stage, and afterwards freely utilised. 



A special point in these early chapters is made of 

 the refraction of light by media of varying density, and 

 the results are used to illustrate and explain the 

 phenomena of mirage. Prof. Wood's arrangement 

 for producing mirage on the lecture table is well 

 worth notice; so too are the experiments on anomalous 

 dispersion, and especially the very beautiful one for 

 showing the anomalous dispersion of sodium vapour. 

 We come next to the interference of light. The 

 NO. 1925, VOL. 74] 



usual elementary account of the phenomena shown 

 by Fresnel's mirrors and by the biprism is given, but 

 it is supplemented with a simple description of how 

 to make a pair of very satisfactory mirrors from a 

 piece of modern mirror glass, or a biprism from some 

 slips of glass and Canada balsam, and we are told 

 that " a prism made in this way works quite as well 

 as those supplied by opticians." 



After a reference to the phenomena of light beats 

 and achromatic fringes we pass on' to the colours of 

 thin plates and Newton's rings. The section on the 

 polarised fringes produced by two streams polarised 

 at right angles is very interesting, so too is that on 

 the preparation of Alms for the exhibition of Newton's 

 colours. 



Diffraction is treated at first in an elementary way, 

 then by means of Cornu's spiral, and finally, for a 

 few simple cases, by means of calculation ; the sec- 

 tions on the grating may be specially commended. 

 In chapter viii. we find an able discussion of the 

 modern interference spectroscopes, and here again, 

 both in the case of the Michelson interferometer and 

 of the echelon grating, the experimental conditions for 

 successful working are carefully discussed. The inter- 

 ferometers of Fabry and Perot, and also of Lummer 

 and Gehrke, are also described. The chapter on 

 double refraction proceeds on ordinary lines. Stokes's 

 verification of Huyghens's law might with advantage 

 have replaced that due to Malus, which cannot give 

 results of great accuracy. In chapter xii., the theory 

 of reflection and refraction, the reader is introduced 

 to Maxwell's equations of the electromagnetic field, 

 which henceforth become his main guide in the theo- 

 retical part of the book, though the theory of dis- 

 persion is in the first instance developed on the lines 

 of the work of Sellmeirr and Helmholtz. 



With regard to optical theories generally, Prof. 

 Wood has from the first adopted the view put for- 

 ward by Schuster in the preface to his recent book 

 on optics. 



" So long as the character of the displacements 



which constitute the waves remains undefined we 



cannot pretend to have established a theory of 



light." 



In dealing with the theory of reflection and refrac- 

 tion, the importance of the part played by surface 

 films is duly noted. From this point onwards the 

 interest of the book becomes greatly increased. An 

 admirable account is given of recent work, much of 

 it due to the author, connected with dispersion and 

 absorption, especially the anomalous dispersion of 

 sodium vapour. The optical properties of metals are 

 developed on the electromagnetic theory, and reference 

 is made to the important work by Rubens and Hagen, 

 who showed that many of the discrepancies between 

 theory and experiment noted by other observers arose 

 from the employment of waves of too short length in 

 the investigation. 



After some account of rotatory polarisation, we come 

 to the chapter on magneto-optics. The theory is 

 worked out according to the two lines indicated by 

 Drude; the first hypothesis is based on the existence 



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