Intelligence and Miscellaneous Articles, 493 



with this light, the two surfaces between which the interferences are 

 produced could be removed to a distance of 15 millimetres, and still 

 the existence of distinctly visible rings was confirmed. The differ- 

 ence between the path of the rays was then about 50,000 undula- 

 tions. Similar observations could be made with plates of glass 

 whose faces are virtually parallel ; and by observing in a perpendicular 

 direction the light reflected from their surfaces, very beautiful inter- 

 ference-fringes could be seen even when the thickness of the glass 

 amounted to 10 millimetres. 



Under these circumstances the temperature is seen to exert a very 

 marked influence on the position of the fringes ; so that, heating the 

 glass a certain number of degrees, the fringes on tbe surface are 

 displaced a quantity proportional to the excess of temperature. 

 This displacement, which may be accurately measured, may be 

 referred to two very distinct actions of heat : first, to expansion, 

 which increases the thickness of the glass and consequently the 

 difference in path of the two rays ; secondly, to a modification in 

 the velocity of light, and hence in the refractive index. But, knowing 

 the coefficient of the expansion of glass and the temperature, we can 

 calculate by how much the fringes ought to be displaced by this 

 cause alone, and then compare the result of calculation with that 

 of observation. 



Thus there are all the elements necessary to decide if the refrac- 

 tive index is modified by heat, and to estimate numerically the modi- 

 fications which may be produced. 



This furnishes a method which may without difficulty be applied 

 to all transparent solid bodies capable of being cut in plates with 

 parallel faces. 



The second part of the memoir contains the results of several 

 series of experiments made with substances of different kinds — 

 crown-glass, ordinary flint glass, a denser variety of flint-glass, 

 fluor-spar, and Iceland spar. 



Thus, it has been observed that the refractive index of ordinary 

 glass increases a little, but very slowly, as the temperature rises — 

 a result which agrees with the previous results of Arago and of 

 M. Neumann. With crown-glass the index did not undergo any 

 apparent change. 



With flint glass there is a very distinct increase of the index ; 

 and with heavy flint the increase is still greater. 



Fluor-spar has the singular property that its refractive index 

 diminishes in a marked degree as the temperature rises ; it is the 

 only solid body, among those hitherto examined, which behaves in 

 this way. This property is common to it, with all liquids and gases. 



Lastly, Iceland spar presented interesting phenomena in reference 

 to the remarkable changes which heat occasions in the form of its 

 crystals and in the intensity of its double refraction, according to 

 the researches of Mitscherlich. The effects produced by heat on 

 the two refractive indices have been capable of being measured, and 

 certain consequences relative to the singular phenomena of expan- 

 sion of this crystal have been drawn from them. — Comptcs Rendus, 

 June 23, 1862. 



