298 On Double Refraction. 



If we expose this plate to polarized light, we shall find that it pos- 

 sesses one axis of positive double refraction, and exhibits the polar- 

 ized tints as perfectly as many crystals of the mineral kingdom. The 

 structure thus communicated to the soft film by pressure does not 

 belong to it as a whole, nor has it only one axis passing through its 

 centre like a circular piece of unannealed glass. In every point of it 

 there is an axis of double refraction perpendicular to the film, and 

 the doubly refracting force varies with the inclination of the incident 

 ray to this axis, as in all regular uniaxal crystals. When the two 

 plates of glass are drawn asunder, we can remove one or more por- 

 tions of the compressed film, and these portions act upon light ex- 

 actly like films of uniaxal mica or hydrate of magnesia, and devel- 

 ope a doubly refracting force of equal intensity. 



This remarkable experiment presents an interesting subject of in- 

 quiry. That the regular double refraction of the film is developed 

 by the agency of pressure cannot be doubted ; but it does not at first 

 sight appear whether it is the immediate efiect of the pressure, or is 

 the same doubly refracting force which produces the quaquaversus 

 polarization that takes place when the resinous film indurates without 

 constraint. In this state of the film the axes of double refraction are 

 clearly turned in every conceivable direction ; and it is impossible to 

 suppose that a pressure in one direction could suddenly arrange all 

 these axes in parallel positions. The double refraction of each par- 

 ticle of the film has therefore been developed by the compressing 

 force similarly applied to them ; and in producing this effect, it must 

 have deprived each particle of the doubly refracting structure which 

 it previously possessed. The substitution of one doubly refracting 

 structure for another may be easily effected in many bodies. Even 

 in regular crystals we can by heat or pressure modify or remove their 

 double refraction. Nay, we can take away one axis from a biaxal 

 crystal, and communicate a second axis to an uniaxal one. When the 

 doubly refracting structure is produced by induration, we can remove 

 it wholly by pressure, and replace it with another even of an opposite 

 character ; and when it is generated by the living principle, as in the 

 case of the crystalline lenses of animals, we can take it away entirely, 

 and substitute a new and more powerful doubly refracting structure 

 by induration. 



We may therefore consider it as clearly established that the uniaxal 

 double refraction of the resinous mass has been communicated to the 

 individual molecules by simple pressure ; the increased transparency 



