It is possible to use the polarization caused by the scattering of light within a photoelastic model in place of the usual analyzer in a photoelastic polariscope. This enables one, in effect, to use an analyzer which lies within the interior of the model, and so make analyses of three‐dimensional stress systems very conveniently. This is accomplished in practice by projecting into the model a beam of polarized light which has been collimated and passed through a slit, thus confining the beam to a thin sheet of light. Such a sheet will illuminate any desired plane section of the model, and in this section will appear interference fringes from which the stresses in the section may be determined. By successive examination of plane sections one may completely investigate the model. In this method the spacing of the fringes is the significant feature, rather than their order, as in previous methods. It is possible, in many cases, to determine the directions of the principal stresses at interior points as well as the maximum shear at such points. Boundary stresses are evaluated both as to direction and magnitude. Equipment for this method differs in constructional details from that employed in plane stress studies in that a source of light of higher intensity must be provided and a slit is used to restrict the beam.
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