
EQUILIBRIUM OF ELASTIC SOLIDS. 97 
Therefore, if the solid be viewed by polarized light (transmitted parallel to 
the axis), the difference of retardation of the oppositely polarized rays at any 
point in the solid will be inversely proportional to the square of the distance from 
the axis of the cylinder, and the planes of polarization of these rays will be 
inclined 45° to the radius at that point. 
The general appearance is therefore a system of coloured rings arranged op- 
positely to the rings in uniaxal crystals, the tints ascending in the scale as they 
approach the centre, and the distance between the rings decreasing towards the 
centre. The whole system is crossed by two dark bands inclined 45° to the plane 
of primitive polarization, when the plane of the analysing plate is perpendicular 
to that of the first polarizing plate. 
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 li¢ht, 
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 maintained by an op- 
posite strain on another part. 
Two other uncrystallised substances have the power of retaining the polarizing 
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 Davin 
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 in- 
* By Dr Waicurt, I believe. 
