THE EQUILIBRIUM OF ELASTIC SOLIDS. 43 



A jelly of isinglass poured when hot between two concentric cylinders forms, 

 when cold, a convenient solid for this experiment ; and the diameters of the rings 

 may be varied at pleasure by changing the force of torsion applied to the interior 

 cylinder. 



By continuing the force of torsion while the jelly is allowed to dry, a hard 

 plate of isinglass is obtained, which still acts in the same way on polarized light, 

 even when the force of torsion is removed. 



It seems that this action cannot be accounted for by supposing the interior 

 parts kept in a state of constraint by the exterior parts, as in unannealed and 

 heated glass ; for the optical properties of the plate of isinglass are such as 

 would indicate a strain preserving in every part of the plate the direction of 

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

 tained by an opposite strain on another part. 



Two other uncrystallised substances have the power of retaining the polariz- 

 ing 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 1830. 



When a compressed plate of this substance is examined with polarized light, 

 it is observed to have no action on light at a perpendicular incidence ; but when 

 inclined, it shews 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 negative. 



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 polarizing 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 extended gutta percha placed between two layers of Canada balsam acts like 



* By Dr Wright, I believe. 



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