( 214 ) 



CHAPTEE XIII. 



INTERFERENCE OF POLARIZED LIGHT. 



(224). HAVING considered the theory and laws of double 

 refraction, we are prepared to study the phenomena of inter- 

 ference which take place when polarized light is transmitted 

 through crystalline substances. The effects displayed in such 

 cases are probably the most splendid in optics ; and when it 

 is considered that, through them, an insight is afforded into 

 the very laboratory of Nature itself, and that we are thus en- 

 abled almost to view the interior structure and molecular 

 arrangement of bodies, the subject will hardly be thought in- 

 ferior in importance to any other in the science. 



The first discoveries in this attractive region were made 

 by Arago, who presented a memoir to the Institute, in the 

 year 1811, on the colours of crystalline plates when exposed 

 to polarized light. The subject has since been prosecuted 

 with unremitting ardour by the first physical philosophers of 

 Europe, and among the foremost by Biot, Brewster, and 

 Fresnel. 



(225) It has been shown (166), that when a beam of 

 light, polarized by reflexion, is received upon a second re- 

 flecting plate at the polarizing angle, the twice-reflected light 

 will vanish, when the plane of the second reflexion is per- 

 pendicular to that of the first. The first reflector, in any 

 apparatus of this kind, is called the polarizing plate, and the 

 second (for reasons which will presently appear), the analyz- 

 ing plate. Now, if between the two reflectors we interpose a 



