Method and apparatus for determining surface birefringences in a transparent material employing a prism place adjacent to the surface

Claim 1

IN AN INSTRUMENT FOR DETECTING AND MEASURING SURFACE STRAINS IN A GLASS SPECIMEN HAVING A PLANE SURFACE, A GLASS PRIMS HAVING A PLANE FACE ADAPTED TO REST ON THE PLANE SURFACE OF THE SPECIMEN IN CONTACT THEREWITH, A LIGHT BARRIER IN SAID PRISM, LYING IN A PLANE NORMAL TO SAID FACE AND INTERSECTING THE SAME IN A FIRST LINE, MEANS FOR PROJECTING A PLANE BEAM OF LIGHT THROUGH SAID PRISM AND ONTO SAID FACE TO INTERSECT THE SAME IN A SECOND LINE CLOSELY ADJACENT SAID FIRST LINE AND AT A DIHEDRAL ANGLE NOT LESS THAN THE CRITICAL ANGLE OF INCIDENCE OF SAID PRISM, AND OPTICAL MEANS FOR OBSERVING LIGHT FROM SAID PROJECTING MEANS REFRACTED INTO SAID PRISM ON THE OTHER SIDE OF SAID BARRIER, AFTER TRAVERSING SURFACE LAYERS OF THE SPECIMEN.