The following curious observation is made by M. Luvini, of Turin :— "When there is a fog between two corresponding stations, so that the one station can with difficulty be seen from the other, if the observer passes a colored glass between his eye and the eye-piece of his telescope, the effect of the fog is very sensibly diminished, so that frequently the signals from the other station can be very plainly perceived, when without the colored glass, the station itself could not be seen. The different colors do not all produce this effect in the same degree. The red seems the most proper for the experiment. Those who have good sight prefer the dark red, those who are short sighted like light red better. The explanation of this effect depends upon the fact that the white color of the fog strikes too powerfully upon the organ of light, especially if the glasses have a somewhat large field."