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.