a fog detector of the split beam transmissometer type is described. two light beams, derived from a single modulated solid state source, are detected at the ends of two separate paths, one enclosed and one through the atmosphere. the outputs from the detectors are switched and combined in such a way that the final output is a direct measure of the attenuation undergone by the fog path beam, and is free from zero drift, the effects of ambient light admitted into the apparatus, and photo-detector leakage currents. the prototype has established that it should be possible to produce an instrument that will determine meteorological visible ranges between 10m and 200m to within an accuracy of 10 per cent, and at a cost of less than 100 when made in quantity. improvements are suggested which in a production model would eliminate difficulties with thermal expansion and contraction which reduced the accuracy of the prototype over the temperature range likely to be encountered in an outside environment. (a)

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