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gunnery and rifle-shooting. The sights and
the object aimed at cannot be in focus to-
gether, and a great deal depends on the
form of sight. Tycho Brahe invented, and
applied to the pointers of his instruments,
an aperture-sight of variable area, like the
iris diaphragm used now in photography.
This enabled him to get the best result with
stars of different brightness. The telescope
169