“…Referring the interested reader to the cited papers for details of the method, we restrict ourselves here to mentioning that it results in the pair of functional equations known as the summation laws for the reflectance and transmittance. Note that the method of addition of layers is closely related to that of the invariance principle and invariant imbedding, therefore the same technique is named differently by different authors (see, e.g., Ambartsumian 1943Ambartsumian , 1960Sobolev 1963Sobolev , 1974Chandrasekhar 1960;Busbridge 1961;Bellman et al 1956Bellman et al , 1960Preisendorfer 1958Preisendorfer , 1976Abhyankar & Fymat 1969, 1970Peraiah 1984).…”