COLOURED GLASSES IN FOGS.
When there is a fog between two places, so that the one station can with difficulty be seen from the other, if the observer passes a coloured glass between his eye and the eye-piece of his telescope, the effect of the fog is very sensibly diminished, so that frequently signals from the other station can be very plainly perceived, when, without the coloured glass, the station itself could not be seen. The different colours do not at 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.