Page "learned" Paragraph 35

The concept of the strain energy as a Gibbs function difference Af and exerting a force normal to the shearing face is compatible with the information obtained from optical birefringence studies of fluids undergoing shear.

Essentially these birefringence studies show that at low rates of shear a tension is present at 45-degrees to the direction of shear, and as the rate of shear increases, the direction of the maximum tension moves asymptotically toward the direction of shear.

According to Philippoff, the recoverable shear S is given by Af where **yc is the angle of extinction.

From this and the force of deformation it should be possible to calculate the elastic energy of deformation which should be equal to the Af calculated from the pressure normal to the shearing face.