There are cases in which interference colour appears on the surface of the glass exposed to the air. It seems that the glass surface is attacked by the water vapour, and is altered in various ways. These facts are presumably related to the problem of the water vapour condensation on the glass surface. As for the scratches on the glass surface, special condensation patterns are observed.We have used the polished surface of the various optical glasses because homogeneous surfaces are obtained easier than other cases. We have observed mostly with naked eyes and in some significant cases (for example, on the various scratches of the glass surface) with the help of microscope, the appearances of dew drops and their growths on the altered portions on the optical glass effected by print of finger patterns, metallic soap films, synthetic resin films, interference films on the glass surface caused by immersing in the HNO3 at various lengths of time, or scratches in various ways, etc.The conclusions of the observations are summarized as follows:(1) Dew drops appear earlier and develop faster on the print of finger patterns (Fig. 2, 3) and metallic soap films (Fig. 4, 5, 6) than on the glass surface proper.(2) Dew drops appear slower on the resin film (Fig. 7, 8, 9, 10) than on the glass surface proper.(3) Dew drops appear in most cases earlier and develop faster on the interference films than on the glass surface proper, but it seems difficult conclude so because of the complexity of its surfaces.(4) Dew drops appear earlier and develop faster on the latent scratches (G. D. D. Growth of dew drops 1, 2, 3, 4) and on the scratches by the razor blade (G. D. D. 5, 6) than on the intact glass surface.(5) As for the scratches, condensation patterns will differ case by case.