Luminescence is a general term which includes the many phenomena involving the absorption of energy by a substance and its re=emission as visible or near visible radiation. Photoluminescence implies excitation 7 by irradiation with electromagnetic radiation; cathodoluminescence implies excitation by electron bombardment; thermoluminescence or "glow" refers to additional excitation by heat of a phosphor previously irradiated at a low temperature; and electroluminescence implies excitation by an electric field. The most recently observed phenomena, and the subjects of this investigation are those included in the general term "electrophotoluminescence" which refers to the effect of an electric field on photoluminescence. Luminescence includes both fluorescence and phosphorescence. Fluoresgence refers-to the emission observed either during excitation or within 10^-8 sec after excitation has ceased. Phosphorescence refers to any omission observed later than 10^-8 sec after excitation has ceased. Most substances are not luminescent but dissipate absorbed energy in heat. A few substances dissipate absorbed energy in photoelectrons, or in other forms. This discussion will be concerned exclusively with that class of crystalline inorganic solids which exhibit phosphorescence due to controlled traces of impurity. These solids are known as phosphors.