234 INTERFERENCE OF POLARIZED LIGHT. 



fracting structure, and those near the centre a negative one. 

 The effects are reversed when a plate of glass, uniformly 

 heated, is rapidly cooled at one of its edges ; and all the ap- 

 pearances vanish when the glass acquires the same tempera- 

 ture throughout. 



If we transmit heat from the surface to the axis of a 

 glass cylinder, by immersing it in heated oil, it will display a 

 system of rings similar to those of a negative crystal with one 

 axis, the axis of the cylinder being also the axis of double 

 refraction. "When the heat reaches the axis the double 

 refraction begins to weaken ; and the colours disappear alto- 

 gether when the glass is uniformly heated. Again, if the 

 cylinder, when in this state, be made to cool rapidly by sur- 

 rounding it with a good conductor of heat, it will transiently 

 assume the opposite character of a positive double-refracting 

 crystal ; and when it is restored to a uniform temperature 

 throughout, all traces of double refraction again disappear. 

 If we employ an elliptic cylinder, instead of a circular one, 

 in the experiment just described, it will exhibit the coloured 

 curves formed by a biaxal crystal : and the phenomena may 

 be endlessly varied by varying the form of the glass to which 

 the heat is applied. 



If now, by any means, the glass be arrested in one of 

 these transient states, it will acquire a permanent double- 

 refracting structure. This has been accomplished by raising 

 it to a red heat, and then cooling it rapidly at the edges. 

 For as the outer parts, which are thus more condensed, as- 

 sume a fixed form in cooling, the interior parts must accom- 

 modate themselves to that form, and therefore retain a state 

 of unequal density. The law of densit}^, and therefore the 

 double-refracting structure, will depend on the external 

 form ; and it is accordingly found that the coloured bands 

 and patches, which such bodies display in polarized light, 

 assume a regular arrangement varying with the shape of the 

 mass. 



