Diesel engine exhaust gas changes in colour between light gray and black according to the state of combustion, the density of colour indicating the amount of contained soot which tells how far the combustion is from completion. In measuring the smoke density, the lenses for observation are generally air-curtained to prevent them from being contaminated. Concerning this device, the :author points out the trouble of exhaust gas being mixed with or its flow rate being disturbed by the curtaining air causing inaccuracy of measurement. The author's method of obtaining the true smoke density and the result of experiment by this method are described.
