EXPERIMENTS, made at the National Physical Laboratory, comparing the revealing power of white and coloured headlight beams in fog, are described by Dr. W. S. Stiles in the Illuminating Engineer of October. Two headlights of known candle-power distribution were mounted at the height and spacing employed on the average car. They were arranged to throw their beams parallel to and in the direction of the road. The observer stood behind the offside headlight and viewed a disc painted grey with its centre lying on the axis of the light. Test discs of different reflecting powers were used in the experi ments. The discs were moved up and down the road, and the distances at which the discs first became invisible were measured. The experiments were repeated with both coloured and neutral light filters held in a frame at a suitable height. Taken as a whole, the results point definitely to the conclusion that the sole effect of the colour filters is due to the reduction in light intensity. This was proved by showing that the range of observations for neutral and colour filters, when plotted against filter trans mission, lay on the same curve. The effects that occurred always showed a reduction in the range when compared with the unmodified beam. So far as the experiments went, the revealing power of a coloured beam is the same as that of a white beam of the same intensity. This conclusion was quite definite for the winter fogs in. Surrey, near the National Physical Laboratory. There is a possibility that the conclusions may not hold good for other kinds of fog.