354 ON THE DISTRIBUTION OF THE POLARISING FORCE 



along the axis of Ice, Quartz, &c. and other crystals of the po- 

 sitive class. 



If the circular plate of glass, on the contrary, receive the po- 

 larising structure transiently, by heating it uniformly in boil- 

 ing oil, and allowing it to cool rapidly, it will exhibit a similar 

 system of rings ; but this system has a negative polarisation, 

 like the rings formed by Calcareous spar, Beryl, &c. and other 

 crystals of the negative class. 



This opposition in the character of the two plates may be finely 

 observed, by combining them together. The resulting system 

 of rings, when two positive or two negative plates are combined, 

 will be the same as that which would have been produced by a 

 plate equal to the sum of their thicknesses ; but when the one 

 is positive, and the other negative, the resulting system will be 

 that which would be produced by a plate equal to the differ- 

 ence of their thicknesses. Hence, when the negative system is 

 exactly equal to the positive system, they will destroy each 

 others effects, and the compound plate will have no action 

 whatever upon polarised light. 



By comparing the value of the tints with their distances from 

 the centre of the plate, I have found, that they vary as the 

 squares of their distances from the axis. Hence if T is the 

 tint which corresponds to any distance D, the tint t correspon- 

 ding to any other distance d, will be found by the formula 



Td 2 





D 2 



2. On Plates of Glass with Two Axes of Polarisation. 



When a plate of glass deviates from the circular form, and is 

 either elliptical or rectangular, it has two axes of polarisation, 

 one of which is perpendicular to the plane of the plate, and 

 the other at right angles to it, and lying in the plane of the 

 plate. When the plate has received the polarising structure 

 transiently, by the transmission of heat, or permanently, by 



being 



