402 



SCIENCE. 



[Vol. VII., No. 169 



intervals indefinitely small compared to a wave- 

 length of light. This suggestion was worked out 

 by Cauchy between 1830 and 1835, and for a long 

 time was supposed to complete the undulatory 

 theory of light. But during the last few years 

 the theory has undergone a very active critical 

 revision by physicists, prompted by two capital 

 discoveries ; namely, the extraordinary relations 

 between the electrical and optical properties of 

 bodies, and the anomalous dispersion of light. 

 Students of physics are well aware that these two 

 discoveries are prompting rapid developments in 

 two distinct lines, — the electro-magnetic and the 

 molecular theories of light. 



This book by Dr. Ketteler is a very important 

 contribution to the subject from the stand-point 

 of molecular dynamics, the problems proposed 

 and solved being much the same as those treated 

 by Sir William Thomson in his lectures at Balti- 

 more in 1884. Starting with Sellmeier's paper of 

 1872, on anomalous dispersion (which establishes 

 certain differential equations closely allied to Bes- 

 sel's differential equation of the motion of a 

 pendulum in air), the author passes in review the 

 theories of Helmholtz, Meyer, and Lommel, and 

 then develops his own, which differs from the 

 others in its assumptions as to the nature of the 

 reaction of the molecules of matter upon the 

 ether. It is well known that the essential fea- 

 ture of these theories is that the molecules of 

 gross matter have, in general, definite periods 

 of vibration comparable to the periods of light 

 waves, and also (since Sellmeier) that they are 

 subject to a ' damping ' effect. As in this treat- 

 ment the absorption of the medium becomes of 

 equal physical importance with its refractive 

 power, Ketteler proposes to define as the law of 

 dispersion the equation containing complex va- 

 riables, expressing both the curve of refraction 

 and the curve of absorption. 



With this basis, the author derives a law of 

 refraction for transparent bodies and those having 

 a single symmetrical absorption band, which con- 

 tains only four constants, and which satisfies 

 observations remarkably well. Even for the flint 

 glass for which Langley has given indices cor- 

 responding to wave-lengths from 2.36 to 0.34 (i.e., 

 for relative wave-lengths varying from one to 

 seven), the formula seems to be wholly ade- 

 quate. This must certainly be regarded as a 

 remarkable feat; but, as the author concludes 

 (p. 445) that he has accounted for all the phe- 

 nomena of light except phosphorescence and fluo- 

 rescence, this alone does not establish the claim of 

 the book to unqualified praise, it is true that 



his treatment leads to the accepted solutions of 

 Fresnel for the phenomena of reflection, refrac- 



tion, and double refraction ; but whether the pro- 

 cesses are strictly legitimate may perhaps rest 

 under some suspicion, in view of the fact that no 

 one, before him at least, has succeeded in estab- 

 lishing a satisfactory theory for all of these phe- 

 nomena on the basis of molecular dynamics. 

 Even Sir William Thomson, in the Baltimore 

 lectures, who approaches the problems from a 

 stand-point not unlike that of Ketteler, except 

 that he dispenses with terms involving viscosity 

 as unphilosophical, emphasizes the statement that 

 double refraction does not yield to the method. 



It is a curiosity worth noting, that the author's 

 theory explains the enormous dispersion of bi- 

 sulphide of carbon, not by the great ' dispersive 

 power ' as defined by the second constant in 

 Cauchy's equation, but by the exceptionally great 

 wave-length of its absorption band, which is cal- 

 culated as equal to 0.220. 



The discussion of the electro-magnetic theory of 

 light is suggestive, and, did it not demand too 

 much space, some of it might well be quoted. 

 This closes the first part of the book. The second 

 part, of about two hundred pages, is devoted to 

 the discussion of the author's experiments to test 

 his theories : they, of course, largely relate to the 

 phenomena of anomalous dispersion. 



THE ROTIFER A. 



We have the pleasure of reviewing a very ex- 

 cellent work, which will be as welcome to the 

 amateur and microscopist as serviceable to the 

 professional zoologist ; for, to judge by the two 

 parts already issued, the monograph of the Rotif- 

 era, by Mr. Hudson and Mr. Gosse, will be ex- 

 cellent throughout. The work is to be in two 

 volumes of three parts each, with over thirty 

 double plates, of which nearly all are to be colored. 

 Its aim is to monograph the known species of the 

 class, giving an improved classification, and in- 

 cluding such anatomical observations as can be 

 made upon the living specimens. 



In accordance with this aim, the first chapter is 

 an outline of the anatomy of the group Brachi- 

 onus rubens, serving as type of the class ; the 

 descriptions, which are clear, being helped out 

 by a plate of fairly good anatomical figures. The 

 chapter is satisfactory, except that Mr. Hudson 

 has indulged in the freak of describing the ex- 

 cretory apparatus, or, as it is often called in view 

 of its homologies, the segmental organs, under 

 the head of ' vascular system.' This is the same 

 surprise to us that it would be to find the kidn< y 



The Rotifera; or, Wheel animalcules. By C. T. Hud- 

 son, assisted by P. H. Gosse, F.lt.S. Parts i. ami li. Lou- 

 don, Longmans, 1H85. H°. 



