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have cross wires in the focus, for use as
a pointer to fix accurately the positions of
stars. An Englishman, Gascoigne, was the
first to use this in practice.
From the all too brief epitome here given
of Kepler’s greatest book, it must be obvi-
ous that he had at that time some inkling
of the meaning of his laws–universal gravi-
tation. From that moment the idea of uni-
213
