517 



considered, a change of sign in the reflected vibration in passing 

 through zero, proves to be merely the separating case between two 

 opposite classes of phenomena. M. Jamin has further compared the 

 results of his observations with the formulae of M. Cauchy, with 

 which they manifest a striking agreement. These formulae contain, 

 besides the index of refraction, a second constant, which M. Jamin 

 calls the coefficient of ellipticity ^ which must be determined for each 

 substance in particular. 



In a later memoir published in the 'Annales de ChirmV for 1851, 

 M. Jamin has extended the same results to liquids, which offer 

 several examples of what had been already perceived in the case of 

 solids ; that though if bodies be arranged in order according to their 

 decreasing refractive indices, the coefficient of ellipticity on the whole 

 decreases, and after vanishing increases again negatively, yet there 

 exist exceptions enough to this rule to show that the coefficient of 

 ellipticity cannot be a function merely of the refractive index, but 

 must depend also upon the nature of the body. 



The preceding volume of the same work contains two other 

 memoirs by M. Jamin ; one on the double refraction of quartz, the 

 other on total internal reflexion ; both affording new proofs of his 

 skill and sagacity. 



In a paper published in the 'Annales de Chimie' for 1852, M. 

 Jamin has shown the influence on the phenomenon of Newton's 

 rings of the reflexion of light polarized in the plane of incidence in 

 the neighbourhood of the angle of maximum polarization, and of the 

 changes of phase which it then undergoes. The paper contains also 

 other results relating to the rings which are worthy of notice, not 

 belonging specially to what takes place near the angle of maximum 

 polarization. 



There are other papers of M. Jamin' s to which time will not 

 permit me to refer; but in the year 1856 he brought before the 

 French Academy an instrument, in which he utilized, in a most happy 

 manner, the fringes which had long before been observed as arising 

 from the interference of some of the many portions into which light 

 is divided by reflexion at the surfaces of two plates of glass of equal 

 thickness. This instrument constitutes an interference-refractometer 

 which unites delicacy with singular convenience. 



In the same communication the author has given some highly 



