The
particular question on which Halley had to represent the Royal
Society may be simply stated. When Tycho Brahe made his memorable
investigations into the places of the stars, he had no telescopes to
help him. The famous instruments at Uraniborg were merely provided
with sights, by which the telescope was pointed to a star on the same
principle as a rifle is sighted for a target. Shortly after Tycho's
time, Galileo invented the telescope. Of course every one admitted
at once the extraordinary advantages which the telescope had to
offer, so far as the mere question of the visibility of objects was
concerned. But the bearing of Galileo's invention upon what we may
describe as the measuring part of astronomy was not so immediately
obvious. If a star be visible to the unaided eye, we can determine
its place by such instruments as those which Tycho used, in which no
telescope is employed. We can, however, also avail ourselves of an
instrument in which we view the star not directly but through the
intervention of the telescope. Can the place of the star be
determined more accurately by the latter method than it can when the
telescope is dispensed with? With our present knowledge, of course,
there is no doubt about the answer; every one conversant with
instruments knows that we can determine the place of a star far more
accurately with the telescope than is possible by any mere sighting
apparatus.
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