REPORT ON PHYSICAL OPTICS. 371 



violated. This is not the case in Fresnel's results, which are in 

 fact derived from that law. 



The phenomena of metallic reflexion remain yet to be noticed 

 in connexion with this division of the science of light. 



The effects produced upon light by reflexion at the surfaces of 

 metals did not escape the scrutiny of Mains. From his first 

 experiments upon the subject, Malus concluded that metals had 

 no effect in polarizing the light. He soon, however, modified this 

 opinion, and found that the phenomenon of polarization was 

 partially produced, the effect increasing to a maximum as the 

 incidence approached a certain angle. But the most instructive 

 mode of studying these phenomena, is to let fall upon the metal- 

 lic reflector a ray polarized in a plane inclined at an angle of 

 45° to the plane of reflexion, and to analyse the reflected pencil 

 by a double refracting prism. Proceeding in this manner Malus 

 found that when the incidence was very small or very great, the 

 reflected ray was still polarized ; while at moderate incidences 

 it was depolarized, and the pencil was divided into two in every 

 position of the rhomb. From these facts Malus concluded that 

 the difference between metals and transparent bodies consisted 

 in this, that the latter reflect all the light which is polarized in 

 one plane, and refract all the light polarized in the opposite 

 plane ; while metals on the other hand reflect light which is 

 polarized in both planes. 



The subject of metallic polarization was next examined by 

 Sir David Brewster ; and his labours on this subject constitute 

 the most important addition which has been recently made to 

 our knowledge of the laws of polarized light*. When light re- 

 flected at a metallic surface is analysed by a double-refracting 

 crystal, it is observed to be partially polarized in the plane of 

 reflexion. The effect is greatest in galena, and least in silver ; 

 and the angle at which it is a maximum is about 74", but varies 

 with the metal. By successive reflexions in the same plane Sir 

 David Brewster found that the proportion of polarized light was 

 increased ; and that by a sufficient number of reflexions the light 

 became, as to sense, wholly polarized in the plane of incidence. 

 The number of reflexions required to produce this effect varied 

 widely in the different metals. 



In order to determine the nature and laws of this phenomenon, 

 it is necessary to examine the effect produced upon polarized 

 light. Adopting, then, the method of Malus, Sir David Brewster 

 found that when a ray of light polarized in the azimuth of 45° 



• " On the Phenomena and Laws of ElHptic Polarization, as exhibited in the 

 Action of Metals upon Light," P/)?7. Trans. 1830. 



2 B 2 



