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crometer to cover a planetary disc, and so
to measure the width covered by the planet.
The Marquis Malvasia, in 1662, described
the network of fine silver threads at right
angles, which he used in the focus, much as
we do now.
In the hands of such a skilful man as Ty-
cho Brahe, the old open sights, even with-
out clocks, served their purpose sufficiently
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