The paper describes the construction and use of a simple photometer consisting of a small photoconductive cell mounted so that it can be moved smoothly across an interference fringe system. The detector width is about 0.005 in. The photoconductive current is measured with a commercial electrometer or with a simple transistorized amplifier. Intensity distributions as measured with the photometer are shown for single, double and triple slits, and for a straight edge. The paper also describes the use of the photometer to illustrate the principle of Michelson's stellar interferometer, by making measurements of the visibility of double-slit fringes as a function of the width of the light source.