The stain and frost which are formed at the surface of some lenses or prisms give the difflculties in using the glass in optical system. They are the abnormal surface layer formed by the surface chemical change.The authors have carried out the investigations on the nature of the surface layer formed by the attack of gases and liquids on the two kinds of heavy crown glasses, one having high, and the other low chemical durability. The Chemical agent used for attacking the glass surface were; i) water vapor, ii) wet CO2 gas, iii) wet HCl gas, iv) wet SO2 gas, v) 1N-HCl solution, vi) 1N-H3PO4 solution, vii) 1N-Na2CO3 solution, viii) 1N-NH4Cl solution.In order to unveil the nature of the surface layer and to know how it was formed the authors have used; a) the observation of the glass surface by phase microscope, b) the measurement of haze value, c) investigation by electron diffraction, d) observation of dew-drops on the surface, e) growth of mould, f) observation of the surface with an electron microscope.The results of the investigations are summarized as follows: It was revealed that the frost is formed by the attack of gases on the glass surface forming, respectively, Ba(OH)2. 8H2O or BaCO3 by water vapor, BaCO3 by CO2, BaCl2⋅2H2O by HCl, BaSO4 by SO2. In this case the haze per cent is comperatively large.By the attack of liquid a thin film having the structure of α-quartz mixed partly with Ba-salt is formed. In this case the haze per cent does not become large although interferrence color is observed. It was concluded that the stain of glass surface is observed when a SiO2-rich film is formed at the surface after the metallic ions were leached out.