The authors point out that "The layers with same temperature are not quite parrallel to the ground and are changable tinder different condition" (i.e. the layers are divided arbitrarily). Under such an assumption the correcting formula for refraction in levelling has been derived concerning the effect of light dispersion. The formula with the precision 10-6 is as follows The necessity and the possibility to develop the two wavelengths laser levelling instrument are suggested and the technical parameter ...