586 



NA TURE 



[February 23, 



Life's history becomes in this way a history of conflict, 

 of which no trace has appeared at any earlier point in 

 natural history. The struggle between individuals has 

 not disappeared, but a struggle within the individual life 

 occurs, which has never been visible in the history of any 

 inferior order of life " (p. 55). 



Another aspect of the rational nature is thus 

 defined : — 



" The difference which severs man from the animals 

 lies beyond the craving, and the cunning, and the con- 

 suming of what has been captured. We trace it in his 

 plans for the day, in his preparation of his weapons, in 

 his survey of the heavens, in his taking of reckonings for 

 direction. He deals with the relations of means to ends ; 

 he utilises past experience in his reflections over what 

 has happened ; he reaches general conclusions " (p. 270). 



Perhaps the finest passage in the book is at p. 287, 

 tracing the moral element in the thought of all kinds of 

 men and all diversities of race, as shown by the sense 

 of wrong and injustice. We can only give here the 

 concluding lines : — 



" To this appeals the criminal in the heart of our surging 

 crowds, placed under arrest, if he should be condemned 

 on insufficient evidence. To this appeals every buyer in 

 the market, defrauded by the thrusting of adulterated 

 goods into his hands. And to this does every gentle one 

 make appeal, defrauded in ways still worse, by false ex- 

 pressions of love, from whose falseness recoils a blighted 

 life, bearing through long and weary years witness to the 

 cruel wrong that has been done. Where, along the 

 devious paths in which man is found, is justice not 

 honoured, at least by outcry against harsh wrongs ? " 



There is much in this volume that will attract readers 

 more disposed towards the esthetical and moral than to- 

 wards the scientific aspects of evolution. Agreeing, as 

 the present writer does, with most of the conclusions of 

 the author, he can but regret that they have not been set 

 forth in a manner more likely to attract scientific 

 readers. A. R. W. 



POINCAR^'S ''TH^ORIE MATH^MATIQUE 



DE LA LUMIERE:' 

 Theorie Mathhnatique de la Lumiere. Par H. Poincard, j 



Membre de I'lnstitut. (Paris : G. Carrd, 1889 and 



1892.) 



THIS work consists of two volumes, the first of which 

 comprises a course of lectures delivered by the 

 author in 1 887-1 888, whilst the second contains a further 

 coursedeliveredin 1891-1892. 



The first volume commences with a discussion of the 

 constitution of the luminiferous ether, in which the latter 

 is regarded as a system of discret molecules in stable 

 equilibrium under the action of molecular forces, and the 

 author finally deduces equations of motion of the same 

 form as those which are furnished by the ordinary theory 

 of isotropic elastic media. He then adopts the hypo- 

 thesis, originally due to Lord Kelvin, that the velocity 

 of propagation of the longitudinal wave is practically 

 zero. The principle of Huygens is next dealt with, and 

 this is followed by a chapter on diffraction. A com- 

 plete discussion of all the difficulties attending the reso- 

 lution of waves would carry us too far, but the author 

 does not appear to be acquainted with the masterly 

 NO- I 21 7, VOL. 47] 



investigation of Sir G. Stokes, or the formula deduced 

 by him, which gives the effect of an element of a plane 

 wave at a distant point, and which enables the unsatis- 

 factory reasoning on which the principle of Huygens 

 depends to be dispensed with. The diffraction of light 

 diverging from a focus is next discussed, and the intensity 

 of light diffracted by a circular aperture or disc is ob- 

 tained in the particular case in which the point of 

 observation is the projection of the centre of the aperture 

 or disc upon a screen ; but no mention is made of Prof. 

 Lommel's able investigation in the general case of an 

 excentric point. A few stock problems relating to the 

 diffraction of parallel rays are also discussed, but nothing, 

 is said about the resolving power of optical instruments^ 

 or the theory of gratings, including Prof. Rowland's in- 

 genious invention of concave gratings. 



Chapter V. commences with the theories which have 

 been proposed to explain the photogyric properties of 

 quartz and certain organic substances, and concludes with 

 an account of some of the theories of ordinary dispersion. 

 This is followed by along chapter which begins with 

 Fresnel's theory of double refraction, and then proceeds 

 to discuss the theories of Cauchy, Neumann, Sarrau and 

 Bousinesq. 



In all these theories the ether is regarded as an 

 aeolotropic elastic medium, and in considering them the 

 author is to be congratulated on having shown no sym- 

 pathy with the small minority who regard the writing 

 down of equations as a foolish process ; but although 

 during recent years much time has been spent in elabo- 

 rating such theories, it may be questioned whether the 

 majority of them have contributed any very substantial 

 addition to scientific knowledge. The theory of the pro- 

 pagation of waves in an aeolotropic elastic medium was 

 rigorously investigated by Green as long ago as 1839; 

 and although a theory of this kind is useful in enabling 

 the mind to form a mental representation of the mechan- 

 ism which is required to produce double refraction, it is 

 well known that Green's theory, and all others of a 

 similar character, fail to furnish a satisfactory explanation 

 of this phenomenon. The principal defects of such 

 theories are, that although most of them lead to Fresnel's 

 wave surface, or to one which is a very close approxima- 

 tion thereto, they require us to suppose that the vibrations 

 of polarized light are parallel instead of perpendicular to 

 the plane of polarization ; and they also fail to give results 

 which explain crystalline reflection and refraction, unless 

 certain additional assumptions of a very questionable 

 character are made. Probably it will not be thought an 

 exaggeration to say, that the only theory of elastic media 

 which satisfactorily explains double refraction is the one 

 which is due to the joint labours of Lord Rayleigh, Lord 

 Kelvin, and Mr. Glazebrook. 



At the commencement of Chapter VII., which deals 

 with reflection, the following statement is made (see 

 p. 320):— 



" La reflexion vitreuse a donnd lieu k trois theories 

 ^galement confirme par I'experience, ce sont celle de 

 Fresnel, celle de Neumann et MacCuUagh et celle de 

 Cauchy." 



The theories of Neumann and MacCullagh depend 

 upon the hypothesis that the density of the ether is the 

 same in all media, and that it is the rigidity which 



