216 INTERFERENCE OF POLARIZED LIGHT. 



is perpendicular to it, and, reaching a maximum, when it 

 is inclined to the plane of primitive polarization at an angle 

 of 45. 



If, on the other hand, the crystal be fixed, and the ana- 

 lyzing plate turned, so as to vary the inclination of the plane 

 of the second reflexion to that of the first, the colour will be 

 observed to pass, through every grade of tint, into the com- 

 plementary colour ; it being always found that the light re- 

 flected in any one position of the analyzing plate is comple- 

 mentary, both in colour and intensity, to that which it reflects 

 in a position 90 from the former. This curious relation will 

 appear more evidently, if we substitute a double-refracting 

 prism for the second reflector ; for the two pencils refracted 

 by the prism have their planes of polarization one coincident 

 with the principal section of the prism, and the other at right 

 angles to it, and are therefore in the same condition as the 

 light reflected by the analyzing plate, with its plane of re- 

 flexion successively in these two positions. In this manner 

 the complementary lights are seen together, and may be easily 

 compared. But the accuracy of the relation is completely 

 established by making the two pencils partially overlap ; for, 

 whatever be their separate tints, it will be found that the part 

 in which they are superposed is absolutely white. 



(227) When laminae of different thicknesses are interposed 

 between the polarizing and analyzing plates, so as to receive 

 the polarized beam perpendicularly, the tints are found to 

 vary with the thickness. The colours produced by plates of 

 the same crystal, of different thicknesses, follow the same law 

 as the colours reflected from thin plates of air , the tints rising 

 in the scale as the thickness is diminished ; until finally, when 

 this thickness is reduced below a certain limit, the colours 

 disappear altogether, and the central space appears black, as 

 when the crystal is removed. The thickness producing cor- 



