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XXIII. Notices respecting New Books. 



Theoretische OjptiJc : gegrundet auf das Bessel-Sellmeiersche Princip. 

 Von Dr. E. Ketteler, Professor an der JJniversitdt in Bonn. 

 Large 8vo. (Pp. 652.) Viewig und Sohn : Braunschweig. 



T)EOEESSOB KETTELEE has done a useful service in thus 

 -*- issuing in a collected form the results of his numerous and 

 important optical papers which have appeared from time to time in 

 Wiedemann's Annalen, and in enabling the reader to grasp, as a 

 whole, the theory which he has put forward. The book treats of the 

 propagation of light in isotropic and crystalline media, of dispersion 

 and absorption, and of the conditions which are satisfied at the 

 surface of two separate media. The theory is based on the mutual 

 reaction between the aether and the particles of the transparent 

 medium through which the light is being propagated. 



Various suggestions as to the form of this reaction have been 

 made by Sellmeier, Helmholtz, Lommel, and Voigt, and the equa- 

 tions of motion to which they lead may be summed up in the 

 forms 



m|f=X+E(a'), ^ 

 € IH' r 0) 



™'^=X'-E(i;,r), 



where m and m! are the densities of the aether and matter, £,£' their 

 displacements, X and X' the forces on the aether and matter, so 

 far as they do not depend on the mutual action between the two, 

 while E(£, £') expresses this action. The main difference in their 

 theories arises from variations in the form assigned to the function 

 E. Ketteler holds that it is impossible, with our present know- 

 ledge, to determine with certainty the form of E, and endeavours 

 to eliminate it from the equations. This he does by considering the 

 system of aether and matter as a whole, and equating the increase 

 of kinetic energy of an element of volume to the work done on the 

 element by the action of the aether and matter external to it. 

 This leads him to the equation 



d 2 E d 2 l' / d'i'\ 

 m 5?- a,! 'dF c =^+SBm'(6r+ S ' 3J-), (2) 



C being a constant, and the symbol 2 being supposed to include 

 all the different kinds of matter elements possible. This expresses 

 what is called by Ketteler Sellmeier's principle. 



But a second equation is required to solve the problem, and in 

 some of his earlier papers the author endeavoured to extract this 

 out of the energy equation. In the book before us he realizes the 

 impossibility of this endeavour, and (p. 88) " combines with the 

 above a second, expressing the special form of action of the matter- 



