92 INTERFERENCE OF LIGHT. 



in which v and v ' are the velocities of light in air, and in the 

 plate, respectively. Hence 



If the displacement be equal to the breadth of n fringes, we 



qj 



have a' - a = n\ ; and since = /*, the refractive index of the 



plate, 



e (n - 1) = n\. 



The amount of the displacement of the fringes, in 

 this important experiment, depends on the thickness of the 

 interposed plate, and on its refractive index ; so that any one 

 of these quantities will be determined when the other two are 

 known. Accordingly, by observing the displacement of the 

 fringes produced by a plate of known thickness, the refractive 

 index of the plate is found. Arago and Fresnel have em- 

 ployed this method to determine the refractive powers of the 

 gases. The method is susceptible of very great precision. 

 By observing the position of the fringes formed by two rays, 

 one of which has passed through air, and the other through 

 a vacuum, Arago has shown that the minutest changes in 

 the refractive power of the air may be observed such, for 

 example, as would arise from a variation of temperature 

 amounting to ^th of a degree centigrade. By the same 

 method it was ascertained that dry air was more refractive 

 than air saturated with moisture, the difference amounting, 

 very nearly, to the millionth of the refractive index. 



In connexion with these results, Arago has shown, that 

 the scintillation of the stars is a phenomenon of interference, 

 due to changes in the refractive powers of portions of the 

 atmosphere, through which different portions of light reach 

 the eye. 



