PURPOSE: To remarkably enhance the moisture resistance by forming a reflection preventing layer on the surface of glass by chemical etching with a supersatd. aqueous silica soln. of hydrosilicofluoric acid and by after-treating the layer with a metallic salt soln.

CONSTITUTION: Glass is immersed in a supersatd. aqueous silica soln. of hydrosilicofluoric acid by an ordinary method to form a reflection preventing layer on the glass surface by chemical etching. When the resulting reflection preventing glass is exposed to a hot humid atmosphere, an increase in the reflectance and a decrease in the transmittance are caused in a relatively short time. To correct the defect, the reflection preventing glass is immersed in a metallic salt soln. such as a soln. of Zn(NO3)2, Pb(NO3)2 or BeCl2. The concn. of the metallic salt soln. is adjusted to about 1%.