VELOCITY OF LIGHT IN AIR. 



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\~ l ) 



V = U~ = d(X-i) ' 



The data for the calculation of these quantities for carbon disulphide 

 are given by Verdet ( Annales de Ghimie et de Physique, (3), vol. Ixix, 

 p. 470). They give 



for the line D, &/V = 1-624, &/U = 1-722, 

 for the line E, &/V = 1-637, k/U= 1-767. 



The quotient of the velocity in vacuo divided by the velocity in 

 carbon disulphide, according to Professor Michelson's experiments 

 with the light of an arc lamp, is 1*76 + '02, which agrees very well 

 with kfU. Another theory, which would make the velocity observed 

 in such experiments V 2 /U (Nature, vol. xxv, p. 52), receives no 

 countenance from these experiments. The value of MJ/V 2 would 

 be about 1'53. Some may think that the experiments on water point 

 in a different direction. Taking our data from Beer's Einleitung in 

 die hohere Optik, 1853, p. 411, we get 



forD, &/V = 1-334, fc/U= 1-352, MJ/V 2 = 1-316, 

 forE, Jfe/V = 1-336, &/U= 1-359, MJ/V 8 = 1-313. 



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

 with &/V, or even with MJ/V 2 , than with &/U, but the differences are 

 here too small to have much significance. 



Theoretische Optik, gegrtindet auf das Bessel-Sellmeier'sche 

 Princip, zugleich mit den experimentellen Belegen. 



Von Dr. E. KETTELER, Professor an der Universitat in Bonn. Viewig 

 und Sohn. Braunschweig, 1885. 



The principle of Sellmeier, here referred to, relates to vibrations of 

 ponderable particles excited by the etherial vibrations of light, and to 

 the reaction of the former upon the latter. The name of Bessel is 

 added on account of his previous solution of a somewhat analogous 

 problem 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 ponderable 

 particles, and to substitute a consistent and systematic new structure 

 for the present conglomerate of more or less disconnected principles." 

 Such a work demands a critical examination, which should not be 



