Now the ether surrounds the atoms of all bodies, but it is
not independent of them. In ponderable matter it acts as if its
density were increased without a proportionate increase of elasticity;
and this accounts for the diminished velocity of light in refracting
bodies. We here reach a point of cardinal importance. In virtue of the
crystalline architecture that we have been considering, the ether in
many crystals possesses different densities, and different
elasticities, in different directions; the consequence is, that in
such crystals light is transmitted with different velocities. And as
refraction depends wholly upon the change of velocity on entering the
refracting medium, being greatest where the change of velocity is
greatest, we have in many crystals two different refractions. By such
crystals a beam of light is divided into two. This effect is called
_double refraction_.
In ordinary water, for example, there is nothing in the grouping of
the molecules to interfere with the perfect homogeneity of the ether;
but, when water crystallizes to ice, the case is different. In a plate
of ice the elasticity of the ether in a direction perpendicular to the
surface of freezing is different from what it is parallel to the
surface of freezing; ice is, therefore, a double refracting substance.
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