February 23, 1893] 



NATURE 



387 



varies ; and it is somewhat surprising that M. Poincard 

 does not appear to be aware of the investigations of 

 Lorenz and Lord Rayleigh, who completely exploded this 

 hypothesis twenty years ago by showing that it leads to 

 two polarizing angles. The weak point in the investiga- 

 tions of most French mathematicians on the subject of 

 reflection and refraction arises from the fact that, in con- 

 sequence of their not having made a careful study of 

 Green's papers and the subsequent developments by Lord 

 Rayleigh and Lord Kelvin, they are unable to deal satis- 

 factorily with the longitudinal or pressural wave. The 

 diflficulties arising from the existence of these waves may 

 be got rid of either by assuming, as Green did, that the 

 ratio of the velocity of propagation of the longitudinal 

 wave to that of the transverse wave is very large, or, by 

 adopting Lord Kelvin's hypothesis, that the above ratio 

 is very small ; but it cannot be too emphatically stated 

 that the existence of such waves must not be disregarded, 

 and that any attempt to ignore them will inevitably end 

 in failure. 



This chapter concludes with a brief account of metallic 

 reflection, in which the author has adopted the equations 

 of motion given by Voigt. The chief difficulty in trying 

 to explain metallic reflection, by the introduction of a 

 viscous term into the equations of motion, is due to the fact 

 that Eisenlohr has shown that for certain metals the 

 pseudo-refractive index is a complex quantity whose real 

 part is negative. 



Turning now to Volume IL, which consists of a further 

 •course of lectures delivered in 1891-1892, we find that it 

 commences with the theory of isotropic elastic media in 

 its ordinary form. Next follows a chapter on the electro- 

 magnetic theory, in which the author confines himself to 

 the case of an isotropic medium, and has given no ac- 

 count of the investigations of Glazebrook on crystalline 

 reflection and refraction, in which it is shown that the 

 intensities of the reflected and refracted waves satisfy 

 the same equations as those deduced many years pre- 

 viously by MacCulIagh from an erroneous theory, but 

 which nevertheless explain the facts in a fairly satisfac- 

 tory manner. M. Poincare assumes that the vector 

 potential satisfies the solenoidal condition ; but although 

 the employment of the vector potential is valuable as a 

 mathematical artifice, its use requires extreme care, in- 

 asmuch as it contains an undetermined quantity ; and I 

 believe it can be proved that in certain cases the 

 solenoidal condition is not satisfied. In the electro- 

 magnetic theory of light this difficulty can always be 

 evaded by eliminating the vector potential from the 

 equations, which is the preferable course to pursue. 



In Chapter V., after discussing ordinary reflection and 

 refraction, the author attempts to construct an electro- 

 magnetic theory of metallic reflection and refraction by 

 taking into account the conductivity. This theory leads 

 to Cauchy's formula:;, but requires that the real part of 

 the pseudo-refractive index should be positive, whereas 

 Eisenlohr has shown that for certain metals these formulae 

 cannot be reconciled with experiment unless the real part 

 is negative. In the case of steel this quantity is positive 

 throughout the whole range of the visible spectrum ; but 

 as thin films of iron, when magnetized, exhibit anomalous 

 dispersion, it is doubtful whether this hypothesis is satis- 

 factory even in the case of steel or iron. 

 NO. 12 I 7, VOL. 47] 



The next four chapters are devoted to the principle of 

 Huygens and to diffraction; and in Chapter X. the author 

 has discussed Von Helmholtz's theory of anomalous dis- 

 persion. The advantage of theories of the class to 

 which that of Von Helmholtz belongs is, that they endeav- 

 our to account for dispersion and absorption by taking 

 into account the mutual reaction between ether and 

 matter, and show that when one or more of the free 

 periods of the vibrations of the matter coincides with one 

 or more of the free periods of the rays of the spectrum, 

 absorption and anomalous dispersion will be produced. 

 By the aid of this theory the absorption produced by so- 

 dium vapour may be accounted for, as well as the 

 anomalous dispersion and selective reflection produced by 

 fuchsine and other aniline dyes. The author has not, 

 however, developed the consequences of this theory as 

 far as might be done. 



It is not unnatural that M. Poincard should have given 

 special prominence to the writings of his own country- 

 men ; his treatise would, however, have been much im- 

 proved had he not confined himself so exclusively to the 

 writings of French mathematicians, but had given a 

 fuller account of the work done by mathematicians of 

 other nationalities. A. B. Basset. 



THE MOTHS OF INDIA. 

 The Fauna of British India, including Ceylon and 

 Burma. Published under the authority of the Secretary 

 of State for India in Council. Edited by W. F. 

 Blanford. "Moths." Vol. i. By G. F. Hampson. 

 (London : Taylor and Francis, 1892.) 



MR. HAMPSON is already favourably known to 

 entomologists by his work on the " Lepidoptera 

 Heterocera of the Nilgiri District," which forms Part viii. 

 of the series of " Illustrations of typical specimens of Lepi- 

 doptera Heterocera in the collection of the British 

 Museum." In the work before us he has undertaken a far 

 more important task ; nothing less than a descriptive 

 handbook of the moths of India, which, when complete, 

 will prove as useful to Indian entomologists as the well- 

 known work on the butterflies of India by Marshall and 

 De Nicdville. 



Hitherto the available information on the moths of 

 India has been scattered over a great variety of books 

 and periodicals, far too numerous and costly to be easily 

 available out of London or Calcutta, and extremely diffi- 

 cult to use satisfactorily, even if accessible. But Mr. 

 Hampson has been given the fullest facilities for examining 

 all the principal public and private collections of Indian 

 moths, from that of the British Museum downwards, 

 and has also made free use of the libraries of the British 

 Museum at South Kensington, which now contain the 

 finest series of entomological books in the world ; and the 

 result is a work which can hardly fail to give an 

 enormous impetus to the collection and study of Indian 

 moths. 



Much attention has been paid to the classification of 

 moths, and the introductory pages are occupied with 

 details of structure, illustrated by woodcuts of parts of 

 the head, antennae, legs, and neuration. 1 his is followed 

 by a |[enetic tree of the families of moths, and by a 



