IN a paper by Dr. V. E. Yarsley published in the Electrician of August 8, an interesting account is given of recent developments in moulding materials, for practical purposes. Perhaps the application to luminous plastics is the one which has attracted the greatest public interest. Having a light switch, door handle or telephone clearly visible is of real practical value for a night emergency in war–time. The layman usually associates luminescence with phosphorus or radium, and the terms luminescence, fluorescence and phosphorescence are often misused. Those substances which convert incident radiations into visible light, and not into heat, as is more usual, are called luminescent. Those materials which emit visible radiations only during the period when the exciting radiation is impinging are said to be fluorescent. The commercial luminescent materials do not belong to the radium family. They are usually metallic sulphides, more particularly those of zinc, calcium, strontium and barium. Mixtures may be used, and in some cases increased luminosity results from the addition of minute quantities of metals.