It is very important that the mirror should be of
truly plane and parallel glass, such as instrument-makers procure; the
index glass of a full-sized sextant is very suitable for this purpose:
there is a loss of power when there is any imperfection in the glass. A
plane mirror only three inches across, reflects as much of the sun as a
globe of 120 feet diameter; it looks like a dazzling star at ten miles'
distance.
To direct the flash of the Mirror.--There are makeshift ways of
directing the flash of the mirror; as, by observing its play on an object
some paces off, nearly in line with the station it is wished to
communicate with. In doing this, two cautions are requisite: first, the
distance of the object must be so large compared to the diameter of the
mirror that the play of the flash shall appear truly circular and exactly
like a faint sun (see preceding paragraph): secondly, be careful to bring
the eye to the very edge of the mirror; there should be as little
"dispart" as possible, as artillerymen would say. Unless these cautions
be attended to very strictly, the flash will never be seen at the distant
station.
[Sketch of arrangement as described below].
An object, in reality of a white colour but apparently dark, owing to its
being shaded, shows the play of a mirror's flash better than any other.
The play of a flash, sent through an open window, on the walls of a room,
can be seen at upwards of 100 yards.
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