64 Scientific Intelligence. 



For purposes of numerical calculation, it will be convenient to 



trausf'orrn these formulae by the use of X for the wave-length in 



vacuo, n for the index of refraction of the medium considered, 



and k for the velocity of light in vacuo, which we shall regard as 



constant, in accordance with general usage. By substitution of 



these letters we easily obtain 



k k d(n\-i) 



— n, 



Y ' TJ- d(\~i) 



The data for the calculation of these quantities for carbon disul- 

 phide are given by Verdet (Annales de Chimie et de Physique, 

 3, lxix, p. 470). They give 



for the line D, k/Y= 1,624, 7e/U= 1,722, 

 for the line E, k/Y= 1,637, k/TJ= 1,767. 

 The quotient of the velocity in vacuo divided by the velocity 

 in carbon disulphide, according to Professor Michelson's experi- 

 ments with the light of an arc lamp, is 1 , 76±'02, which agrees 

 very well with Jc /XJ. Another theory, which would make the 

 velocity observed in such experiments V 2 /TI {Nature, xxv, p. 52), 

 receives no countenance from these experiments. The value of 

 &TT /V 2 would be about 1*53. Some may think that the experi- 

 ments on water point in a different direction. Taking our data 

 from Beer's Einleitung in die hohere Optik, 1853, p. 411, we get 



forD, k/Y= 1,334, &/U= 1,352, kTJ/Y 2 = 1,316, 



forE, Jc/Y= 1,336, k/U= 1,359, &U/V a = 1,313. 



The number obtained by experiment was 1,330, which agrees 



better with k/Y, or even with &TJ/V 2 , than with Jc/TJ, but the 



differences are here too small to have much significance. 



J. TV. G. 



9. TheoretiscJie Optik, gegrundet auf das Bessel-Sellmeier''- 

 sche Princip, zugleich mit den experimentellen Belegen. Von 

 Dr. E. Ketteeer, Professor an der Universitat in Bonn. 8°, pp. 

 652. Braunschweig, 1885. Viewig und Sohn. — The principle of 

 Sellmeier, here referred to, relates to vibrations of ponderable par- 

 ticles excited by the etherial vibrations of light, and to the reac- 

 tion of the former upon the latter. The name of Bessel is added 

 on account of his previous solution of a somewhat analogous prob- 

 lem relating to the pendulum. The object of this work is "to 

 treat theoretical optics in a complete and uniform manner on the 

 new foundation of the simultaneous vibration of etherial and pon- 

 derable particles, and to substitute a consistent and systematic 

 new structure for the present conglomerate of more or less dis- 

 connected principles." Such a work demands a critical examina- 

 tion, which should not be undertaken from any narrow point of 

 view. Any faults of detail will be readily forgiven, if the author 

 shall give the theory of optics the nov o~rc5 which it has sought 

 so long in vain. We may add that if this effort shall not be 

 judged successful by the scientific world, the author will at least 

 have the satisfaction of beiug associated in his failure with many 

 of the most distinguished names in mathematical physics. 



