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M. Jamin has for many years been engaged in a series of import- 

 ant experimental researches in optics. One of his earliest labours 

 relates to metallic reflexion. Malus found that metals form an 

 exception to the general rule of polarizing light at a proper angle 

 of incidence, though they do produce a partial polarization. We 

 owe to Sir David Brewster the discovery of the curious modifications 

 which reflexion at the surface of a metal is capable of producing on 

 polarized light. The theory of undulations enables us to form a 

 clear idea of the nature of the light so reflected, and shows, more- 

 over, that in order to know everything about the reflected light, 

 whatever be the nature of the incident light, it is sufficient to know 

 for each angle of incidence three things ; the proportion in which 

 light polarized in each of the two principal planes is reflected, and 

 the difference of phase produced by reflexion. The subject of 

 metallic reflexion had afterwards been considered in its bearings on 

 certain phenomena ; but there were still wanting, on the one hand, 

 an extensive series of careful measures of the three quantities above 

 mentioned, taken for various incidences and for different metals ; and 

 on the other, a theory which should embrace them all, expressing 

 them as functions of the angle of incidence by formulae containing a 

 certain number of constants depending on the nature of the metal. 



M. Jamin applied himself to supply the first of these desiderata; 

 and in an important paper published in the 'Annales de Chimie' for 

 1847, he has given tables containing the results of careful measures 

 taken on several metals. The methods employed cannot here be 

 described, but they exhibited in a remarkable manner a combination 

 of experimental care and skill with a thorough acquaintance with 

 the principles of the theory of undulations. It belongs to the 

 physical mathematician to supply the requisite formulae, which, 

 while differing from mere empirical formulae in being deduced from 

 a physical theory, shall accurately represent the results of observa- 

 tion. 



The subject of metallic reflexion had engaged the attention of 

 mathematicians, and M. Cauchy had arrived at certain formulae to 

 express everything relating to the metallic reflexion of homogeneous 

 light. These formulae contain only two arbitrary constants, depend- 

 ing upon the nature of the metal, and capable of being determined 

 by the observation of two angles. M. Jamin himself has compared 



