208 Professor Airy on the Double Refraction of Quartz. 



that from the suddenness of the condensation of the air, the heat 

 evolved by that condensation has not time to escape, and the 

 elasticity is therefore greater than if it had been slowly con- 

 densed : that, in fact, the law of elasticity is altered. Now the 

 conjecture which I have to offer is, that perhaps there may be 

 in refracting - media something depending on time which alters 

 their elasticity, in the same manner in which heat alters the 

 elasticity of the air: that as in air the elasticity is greater with 

 a quick vibration of particles than it would be if the vibration 

 were exceedingly slow, so also in the refracting media, the elas- 

 ticity may be greater with a quick vibration, than with one some- 

 what slower. Or perhaps the contrary effect may follow : if the 

 vibration be quick, the latent heat (or whatever it is) may not 

 have time to come to the exercise of its influence on the elas- 

 ticity. In the latter case the elasticity, and consequently the ve- 

 locity of transmission, would be greatest for the slowest vibra- 

 tions (that is for the red rays) and therefore they would be the 

 least refracted. I am not prepared to say whether the general 

 law of superposition of small vibrations would hold on this sup- 

 position . 



G.B. AIRY 



Observatory, 

 April 13, 1831. 



