Experiments in mechanical and optical coincidence in photoplasticity

In the extension of the shear-difference method to the separation of interior principal stresses in the elasto-plastic state1, a basic question arises whether the optical isoclinics give the directions of the principal stresses, i.e., whether optical coincidence exists. Experiments are described aiming to answer this question, and preliminary results are given for cellulose nitrate as a model material. Experiments are also described dealing with mechanical coincidence.