ABSTRACT
An isotropic medium subject to a vectorial or a tensorial action in a fixed direction, acquires anisotropy which manifests itself by birefringence. The medium acquires the properties of a uniaxial crystal, positive or negative in different cases, the optic axis of which has the direction of the applied action. Isotropic system subject to actions which render it birefringent. Transparent objects can easily be made from sheets of organic glasses of 1 cm thickness. These objects can be subject to different stresses. The process of tempering is the rapid cooling of molten glass, the exterior parts of which become rigid and produce tensions and compressions in the mass which acquires a permanent birefringence. The existence of a uniform velocity gradient in a liquid consisting of stretched molecules may produce an orientation of the molecules and a birefringence.
