INTERFERENCE OF POLARIZED LIGHT. 231 



fore as the numbers of the natural series for the successive 

 dark bands. For different plates of the same substance, the 

 constant is inversely as the thickness. 



The diagrams above given represent the systems of rings 

 in a biaxal crystal whose axes form a small angle with one 

 another, in two positions of the crystalline plate, the planes 

 of polarization of the polarizing and analyzing plates being 

 at right angles. 



The form of the dark brushes, which cross the entire sys- 

 tem of rings, is determined by the law which governs the 

 planes of polarization of the emergent rays. There is no dif- 

 ficulty in showing, on the principles of Fresnel's theory, that 

 two such dark curves, in general, pass through each pole ; 

 and that they are rectangular hyperbolas, whose common cen- 

 tre is the middle point of the line which connects the pro- 

 jections of the two axes. 



(237) The phenomena of depolarization and of colour, 

 impressed by double-refracting substances upon the trans- 

 mitted light, are, we have seen, the necessary results of the 

 interference of the two pencils into which the light is divided 

 within them. These properties, therefore, become distinctive 

 characters of the double-refracting structure ; and thus enable 

 us to discover the existence, and to trace the laws of that 

 structure, even in substances in which the separation of the 

 two pencils is too minute to be directly observed. By such 

 means it has been discovered that a double-refracting struc- 

 ture may be communicated to bodies which do not possess it 

 naturally, by mechanical compression or dilatation. Thus Sir 

 David Brewster observed, that when pressure was applied to 

 the opposite faces of a parallelepiped of glass, it developed 

 a tint in polarized light, like a plate of double-refracting 

 crystal ; and the tint descended in the scale as the pressure 



