in the Velocity of Propagation of Light in Watsr. 265 



most brilliant lines (a, /3, 7 according to Plucker) of the spec- 

 trum of incandescent hydrogen gas. Landolt's results, in the 

 main, confirm those of Gladstone and Dale, but are not suffi- 

 cient for our purpose, because they are not extended over a suffi- 

 cient range of temperature. 



Fizeau*, by employing the method of interference, measured 

 the alteration in the rate of propagation in glass, fluor-spar, 

 Iceland-spar, and afterwards in rock-crystal; and his experi- 

 ments are of the greatest importance and admirable in regard 

 to their execution. Fizeau, however, in his method confined 

 himself to observations of the homogeneous light of the sodium- 

 flame ; so that from them no conclusion can be drawn concern- 

 ing an alteration in the dispersion. Since these experiments 

 are of great importance, and furnish the only hitherto given 

 numerical data for the alteration of the velocity of propagation 

 in solid bodies, I may be allowed to describe them briefly here. 

 If we denote the index of refraction for the ordinary tempera- 

 ture by n } that for increased temperature by n' ; further, if v 

 denote the velocity of light in air, v' its velocity in the cold, and 

 v n in the warmed body, and if we put 



then 



vP = v!(l+/8t), 



0= 



n—n' 



The following are the results : — 



Substance. 



Glass from St. Gobains 

 Glass from St. Gobains 



second sort 



Crown glass 



Fluor-spar 



Flint glass (common) .. 



Flint glass (heavy) 



Iceland-spar (cut paral- ] 



lei to the axis) J 



Density. 



2-438 



2-514 



2-626 

 3-2 

 3-584 

 4-14 



2-723 



1-5033 

 1-528 



1-5204 



1-435 



1-6112 



1-682 

 1-65850 

 e 1-46835 



(n'-w), 



100£. 



0000163 

 0-0000997 



•0-000108 

 0-000065 



varies irregularly. 



-O-0O136 

 0-00026 

 0-000687 

 +0-0000565 

 4-0-00108 



+0-00049 



-000163 



-0-000408 



-0-0000341 



-0-000727 



Fizeau's experiments therefore give, with the exception of 

 fluor-spar, the following result, that, in spite of the diminution 

 of density, the velocity of the propagation of light diminishes, and 

 therefore the refractive power increases as the temperature increases. 



* " Recherches sur la modification que subit la vitesse de la lumiere 

 dans le verre et plusieurs autres corps sous l'influence de la chaleur," 

 Comptes Rendus, vol. lv. pp. 1237-1239. 



