Compass for use at night.--The great majority of compasses are well-nigh
useless in the dark, that is, when it is most important to be able to
consult them. They are rarely so constructed, that the difference between
the north and south sides is visible by moonlight or by the light of a
cigar or piece of tinder. The more modern contrivances are very
effective; in these the southern half of the compass card is painted
black, the northern being left white. With a very faint light, this
difference can be appreciated. In compasses consisting simply of a
needle, the north end of the needle should have a conspicuous arrow-head.
It is extraordinary how much the power of seeing a compass or a watch at
night is increased by looking nearly at it through a magnifying-glass.
Thus, young people who can focus their vision through a wide range may be
observed poring with their eyes close to their books when the light
wanes. So again, at night-time, a placard, even in large type, is
illegible at a short distance, but easily read on approaching it. It
seems, in order that a faint image on the retina should be appreciated by
the nerves of sight, that image must have considerable extent.
Moonlight or the light of a cigar may be condensed on the compass by a
burning glass, or other substitute for it. (See "Burning Glass.")
True and Magnetic Bearings.--The confusion between true and magnetic
bearings is a continual trouble, even to the most experienced travellers.
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