492 JAMES CLERK MAXWELL. 



properties of the plate of isinglass are such as would indicate a 

 strain pressing in every part of the plate in the direction of the 

 original strain, so that the strain on one part of the plate cannot 

 be maintained by an opposite strain on another part. 



Two other uncrystallised substances have the power of re- 

 taining the polarising structure developed by compression. The 

 first is a mixture of wax and resin, pressed into a thin plate 

 between two plates of glass, as described by Sir David Brewster 

 in the Philosophical Transactions for 1815 and 1850. 



When a compressed plate of this substance is examined with 

 polarised light, it is observed to have no action on light at a per- 

 pendicular incidence ; but when inclined it shows the segments 

 of coloured rings. This property does not belong to the plate as 

 a whole, but is possessed by every part of it. It is, therefore, 

 similar to a plate cut from a uniaxal crystal perpendicular to the 

 axis. 



I find that its action on light is like that of a positive crystal, 

 while that of a plate of isinglass, similarly treated, would be 



The other substance which possesses similar properties is 

 gutta-percha. This substance in its ordinary state, when cold, 

 is not transparent even in thin films; but if a thin film be 

 drawn out gradually, it may be extended to more than double 

 its length. It then possesses a powerful double refraction, which 

 it retains so strongly that it has been used^for polarising light. 

 As one of its refractive indices is nearly the same as that of 

 Canada balsam, while the other is very different, the common 

 surface of the gutta-percha and Canada balsam will transmit one 

 set of rays much more readily than the other, so that a film of ex- 

 tended gutta-percha placed between two layers of Canada balsam, 

 acts like a plate of nitre treated in the same way. That these 

 films are in a state of constraint may be proved by heating them 

 slightly when they recover their original dimensions. 



Some pieces of gutta-percha mounted in this way by 

 Professor Maxwell are still preserved in the Cavendish 

 Laboratory ; as are also the original plates of isinglass above 

 referred to. The interior cylinder employed for twisting 

 these plates was a cork, and in one of the plates two 

 corks were placed with their circumferences about in. 

 apart, and were twisted equally in the same direction. The 

 result of this operation is described as case XIII. in the 

 paper in question, and is determined geometrically by the 



