Theory of the Equivalent of Refraction. 177 



wandte Physik tier Krystalle*, I have in Chap. XIX. § 131, dis- 

 cussed the action of conducted heat upon light. 



The developments carried out on pp. 352-354 are based essen- 

 tially on the following fundamental propositions. 



On the accession of free external heat, an alteration takes 

 place in the length of the molecular paths and the rate at which 

 they are traversed ; hence an alteration in the volume and 

 temperature. The changes in these two (length and rate), how- 

 ever, do not take place symmetrically, and they may have the 

 same or opposite signs. In the latter case a contraction may 

 take place on increasing the temperature of a body, or an expan- 

 sion on cooling it. 



We are taught by Fizeau's observations that in the case of 

 certain solid bodies the indices of refraction increase even when 

 the temperature increases. My calculations of the refractive 

 power of liquid bodies show that the refractive indices diminish 

 more rapidly than is due to the alteration in volume indicated 

 by the temperature. 



For the explanation of this fact we must assume that the 

 amount of heat entering the body has a twofold effect. The 

 change of volume corresponds to a change in the index of re- 

 fraction, which corresponds to the alteration in density (formula 3). 

 An increase of the temperature (that is, an alteration in the mean 

 molecular velocity), will, by increasing the molecular velocity f, 

 cause a diminution in the usual retardation of the luminous im- 

 pulse in those cases where (as in gases and liquids) the molecules 

 are in unstable equilibrium. In support of this is the fact that, 

 according to my calculations in liquid bodies, the refractive power 

 diminishes with an increase of temperature. This action of the 

 temperature on the retarding force appears to be proportional 

 to the variations of temperature, and the light-retarding forces 

 X„ of the substance. (Compare my Lehrbuch der Physik der 

 Krystalle.) 



In contradistinction to gases and liquids, the molecules of solid 

 bodies have no free motion, but are bound together in constant 

 equilibrium. An elevation of temperature and increase of mole- 

 cular velocity cannot, therefore, in all cases be regarded as assist- 

 ing the work and motion performed by the rays of light. 



Although, on the one hand, in part an increase and in part a 

 decrease of the light-retarding forces X„ of the solid body may 

 correspond to an increase of temperature, still, on the other 

 hand, from the fact that in Iceland spar the one index of refrac- 



* Schrauf, Lehrbuch der physik alischen Mineralogie, vol. ii. Lehrbuch 

 der angewandten Physik der Krystalle. Wien, Braumiiller, 1S6S, p. 426. 



t Compare Schrauf, Physik der Krystalle, p. 145. The propagation of 

 the wave impulse. 



Phil. Mag. S. 4. Vol. 36. No. 242. Sept. 18G8. N 



