As many motorists use headlights giving coloured lights, an authoritative and scientific statement as to whether coloured light is better than white light for night driving or during fog has for long been desired. A report by an illumination committee of the Department of Scientific and Industrial Research (Technical Paper No. 20. London: H.M. Stationery Office) has now practically settled the question. Adequate evidence is given that in conditions of slight or thick fog the range of visibility of objects seen in the beam of the headlight is not increased by the use of coloured light obtained from the original white light by means of a filter. One investigator has put on record that in clear weather the range of visibility of an object is increased about 6 per cent by the use of a yellow filter. This result was obtained at a distance of about 900 feet ; but at shorter distances, at which the motorist is more concerned to see objects, the advantage of the yellow filter, in any event small, is still smaller. There is experimental evidence that the power of the to perceive contrasts of brightness in the presence of a dazzling light is enhanced if similar colour filters are placed over the dazzling light and over the light illuminating the objects viewed. The evidence as to whether the use of coloured light obtained from white light by means of a coloured filter enables the eye to detect contrasts of brightness easily is conflicting. Recent investigations have shown that there is a slight advantage, but this is meonsistent with the measurements of earlier workers. There is evidence for a slight increase in the power of the eye to perceive the details of a pattern in black and white' by the use of yellow light obtained from white light by means of a filter.