Several new embedding media of the same refractive index as are described which make possible the microscopic examination of the interior of electroluminescent particles taken from efficient industrial lots. Light is emitted in form of lines or striations extending through most of the diameter of the particles. The brightness of single lines has been measured in dependence of applied voltage, frequency, phase, and position along the line. Evidence is presented that the EL lines are associated with invisible copper‐sulfide‐decorated, conducting imperfection lines of submicroscopic diameter, and that coarse, visible imperfections are not the cause of EL.