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 effect 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 ther 
double refraction. Nay, we can take away one axis from a biaxal 
crystal, and communicate a second axis to an unjawsl one. When the 
doubly refracting structure is produced by induration, we ca remove 
it wholly by pressure, and replace it with another even of an opposite 
as in the 
case of the crystalline lenses of animals, we can take it away enti 
and substitute a new and more powerful doubly refracting 
by induration. ; 
We may therefore consider it as clearly established that the yniaxal 
double refraction of the resinous mass has been communicated to tne 
individual molecules by simple pressure ; the increased transparency 
