October i8, 1900] 



NA TURE 



595 



migration, and he dwells on the risk to parents involved 

 in the process. 



" Long journeys are hazardous. Every Californian 

 salmon which enters upon the long journey to the breed- 

 ing ground is destroyed, and the whole race is wiped out 

 of existence for the good of generations yet unborn. 

 Very few shad ever return to the ocean, and storm and 

 accident and ruthless enemies work their will on the 

 migrating birds and decimate them without mercy ; yet 

 the dangerous return to safe breeding grounds still goes 

 on, in order that children which are yet unborn may 

 survive to produce children in their turn." 



Want of space prevents any further criticism of this 

 most interesting volume. Enough has been said to prove 

 that all the lectures demand the serious consideration of 

 every student of evolution. 



It is a peculiar pleasure to the British naturalist to 

 find the Darwinian principle illustrated and defended 

 with such remarkable force and success by a distinguished 

 American zoologist. E. B. P. 



A MODERN TEXT-BOOK OF OPTICS. 

 Lehrbuch cier Optik. Von Dr. Paul Drude, Professor des 



Physik au des Universitat Giassen. Pp. xiv + 498. 



(Leipzig: Verlag von S. Hirzel, 1900.) 

 pROF. DRUDE'S name is well known to English 

 physicists. As a careful and exact worker, the author 

 of a book on the Physics of the Ether, and the suc- 

 cessor of Gustav Wiedemann in the editorship of the 

 Annalen des Physik, he has already made a high reputa- 

 tion for himself, and the book now under consideration 

 will serve to add to it. Text-books of optics, it is true, 

 are numerous, and the reviewer is apt to think that of the 

 making of many books there is no end. Prof Drude's 

 book, however, contains much that is novel— at any rate, 

 to English text-books — and the student will find up-to- 

 date information on many points of interest. 



In some respects the book has much in common with 

 the late Prof. Preston's well-known text-book ; it gains, 

 however, in the end as a treatise on the subject by the 

 definite adoption of the electromagnetic theory, although 

 it is, of course, in consequence, less complete in that it 

 gives no account of elastic solid theories. 



The first hundred pages deal with geometrical optics. 

 After a clear statement of the fundamental laws, includ- 

 ing the law of the minimum path, and Malus' law of 

 orthotomic systems, we have a chapter on the geometrical 

 theory of optical images. A definition of an optical image 

 is given ; it is then shown that the image of a plane is a 

 plane, and hence the analytical relation between the 

 position of a point and its image is found. From this, 

 following Abbd and Czapski, the geometrical theory of 

 a perfect image is developed clearly and concisely. 

 Throughout this part the book runs on similar lines to 

 Ur. Moritz von Rohr's " Geschichte des Photograph- 

 ischen Objectivs," recently reviewed in these pages 

 (Nature, vol. Ixi. p. 511), though, of course, the more 

 technical part is dealt with much more briefly than in 

 Dr. von Rohr's book. 



Further chapters deal with the formation of images by 

 real rays and the effects produced by the limitations in 

 the size of the pencils in the case of actual instruments. 

 NO. 161 6, VOL. 62] 



The chapter on optical instruments is perhaps rather brief, 

 but it is not the main object of the author to describe 

 these. Throughout this part the book is very different 

 from anything yet published in English, and will well repay 

 study ; it is interesting to read and clearly written ; at 

 the same time, it is commendably brief, and contains little 

 long or cumbersome analysis. 



The remaining four hundred pages are devoted to 

 physical optics. In the first section of this, which deals 

 with the general properties of light, there is, with one 

 exception, nothing particularly novel. The treatment of 

 interference, diffraction, the geometrical theory of double 

 refraction and the colours of polarised light follow the 

 usual lines ; it could hardly be otherwise. The whole is 

 brought up to date, however ; there is, for example, an 

 excellent account of Michelson's echelon spectroscope, 

 while the theory of the resolving power of an optical 

 instrument is given in some fulness ; it is all well done, 

 though the English reader will not find much to make 

 him prefer the book, as a text-book, to Preston. The 

 one exception is the chapter on Huyghens' principle. 

 In his elementary discussion on the rectilinear propaga- 

 tion of light, Dr. Drude makes a distinct step by adopt- 

 ing the methods given by Dr. Schuster {Phil. Mag., 

 vol, xxxi. 1 891), while he completes the discussion by 

 giving Kirchhoff and Voigt's solution of the problem of 

 finding the disturbance at a given point due to disturb- 

 ances existing at some previous time over a surface 

 surrounding the point. To do this, he has, of course, 

 to make use of the differential equation satisfied by the 

 disturbances, and this is not found till a later stage in 

 the book ; but the student who has read sufficient 

 mathematics to follow the proof will probably be ac- 

 quainted with the fact that the differential equation 

 quoted does represent wave motion, and will not find 

 any logical difficulty in the order adopted, while the 

 proof will put the whole theory of diffraction before him 

 on a sounder basis. An English reader, however, who 

 realises what he owes to Stokes in this matter, may be 

 allowed to express surprise that there is no reference 

 in Prof. Drude's work to the great paper on the dynamical 

 theory of diffraction, published in 1849 in the ninth 

 volume of the Transactions of the Cambridge Philo- 

 sophical Society. 



The second section of this part deals with the optical 

 properties of bodies, and here the distinctive points of 

 Prof Drude's method show themselves. After a brief 

 reference to the elastic solid theory of the ether and the 

 difficulties to which it leads, he adopts formally the 

 electromagnetic theory. 



The optical disturbance at any point through which 

 light-waves are passing can be represented by the periodic 

 variations of a vector quantity, the light-vector, as Drude 

 calls it, and in a transparent isotropic medium this vector 

 follows the same laws as do the electric or magnetic 

 force in an insulating body. The electromagnetic theory 

 of light identifies the light vector either with the electric 

 or the magnetic force, Drude adopts the first of the two 

 alternatives. 



In an aeolotropic medium, a third vector, the rate of 

 change of the electric displacement, or the electric 

 current, needs to be considered— in an isotropic body 

 this coincides in direction with, and is proportional to 



