INTERFERENCE OF POLARIZED LIGHT. 215 



plate of any double-refracting substance, the capability of re- 

 flexion at the analyzing plate is suddenly restored, and a por- 

 tion of the light is reflected, whose quantity depends on the 

 position of the interposed crystal. The light is thus said 

 (though improperly) to be depolarized by the crystal ; and it 

 was by this property that the double-refracting structure was 

 detected by Malus in a vast variety of substances, in which 

 the separation of the two rays was too small to be directly 

 perceived. 



In order to analyze this phenomenon, let the crystalline 

 plate be so placed as to receive the polarized beam perpendi- 

 cularly, and let it be turned round in its own plane. We 

 shall then observe that there are two positions of the plate in 

 which the reflected ray vanishes, just as if no crystal had 

 been interposed. These two positions are at right angles to 

 one another ; and they are those in which the principal sec- 

 tion of the crystal coincides ivith the plane of the first reflexion, 

 or is perpendicular to it. When the plate is turned round 

 from either of these positions, the light gradually increases ; 

 and it becomes a maximum, when the principal section is 

 inclined at an angle of 45 to the plane of the first reflexion. 



(226) In these experiments the reflected light is white. 

 But if the interposed crystalline plate be very thin, the most 

 gorgeous colours appear, which vary with every change of 

 inclination of the plate to the polarized beam. 



Mica and sulphate of lime are very fit for the exhibition of 

 these beautiful phenomena, 'being readily divided by cleavage 

 into laminae of extreme thinness. If a thin plate of either of 

 these substances be placed so as to receive the polarized beam 

 perpendicularly, and be then turned round in its own plane, 

 the tint does not change, but varies only in intensity ; the 

 colour vanishing altogether when the principal section of the 

 crystal coincides with the plane of primitive polarization, or 



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