226 INTERFERENCE OF POLARIZED LIGHT. 



,0 O 



r , in which o - e is the interval of retardation of the two 

 A 



pencils. This interval is proportional to the thickness of the 

 crystal. Hence, when that thickness is increased, so that the 

 interval o - e is augmented by a whole number of undula- 

 tions, the intensity resumes its original value. 



(234) It has been stated (226) that the phenomena of co- 

 lour are only produced when the crystalline plate is thin. In 

 thick plates, where the difference of phase of the two pencils 

 contains a great many wave-lengths, the tints of different 

 orders come to be superposed (as in the case of Newton's 

 rings where the thickness of the plate of air is considerable), 

 and the resulting light is white. The phenomena of colour may 

 however be produced in thick plates, by superposing two of 

 them in such a manner, that the ray which has the greater ve- 

 locity in the first shall have the lesser velocity in the second. 

 "We have only to place the plates with their principal sections 

 perpendicular or parallel, according as the crystals to which 

 they belong are of the same, or of opposite denominations. 

 Thus, if the crystals be uniaxal, and both positive, or both 

 negative, they are to be placed with their principal sections 

 perpendicular ; and, on the other hand, these sections should 

 be parallel, when one of the crystals is positive, and the other 

 negative. The reason of this is evident. 



(235) Let us now consider the effects produced when a 

 converging or diverging pencil of rays traverses a uniaxal 

 crystal, in various directions inclined to the axis at small 

 angles: and let us suppose, for simplicity, that the crys- 

 talline plate is cut in a direction perpendicular to the 

 axis. 



Let ABCD be the plate, and E the place of the ,eye. 



