An engineering prototype model of a petrographie microscope for lunar and planetary missions has been designed and fabricated. It is designed primarily for remote operation on soft-landing spacecraft, but it could be adapted for support of a manned mission. An aggregate of crushed rock particles is thermally encapsulated between two opposing sheets of a clear isotropic thermoplastic which has a refractive index of 1.54. The images are projected through refracting lenses and an eyepiece onto the faceplate of a television camera. A television picture is taken both below and above the place of correct focus as well as in that plane for each field of view in order to change the Becke line positions. Each particle is viewed in both plane-polarized and cross-polarized light; the spectral width through an interference filter is small enough to provide light and dark interference rings on a black-and-white television.