then in use. He
was led to this invention by his experiments with light and colors.
In 1671 he presented to the Royal Society a second and somewhat larger
telescope, which he had made; and this type of instrument was little
improved upon until the introduction of the achromatic telescope,
invented by Chester Moor Hall in 1733.

As is generally known, the element of accurate measurements of time
plays an important part in the measurements of the movements of the
heavenly bodies. In fact, one was scarcely possible without the other,
and as it happened it was the same man, Huygens, who perfected Kepler's
telescope and invented the pendulum clock. The general idea had been
suggested by Galileo; or, better perhaps, the equal time occupied by the
successive oscillations of the pendulum had been noted by him. He had
not been able, however, to put this discovery to practical account. But
in 1656 Huygens invented the necessary machinery for maintaining the
motion of the pendulum and perfected several accurate clocks. These
clocks were of invaluable assistance to the astronomers, affording as
they did a means of keeping time "more accurate than the sun itself."
When Picard had corrected the variation caused by heat and cold acting
upon the pendulum rod by combining metals of different degrees of
expansibility, a high degree of accuracy was possible.

But while the pendulum clock was an unequalled stationary time-piece, it
was useless in such unstable situations as, for example, on shipboard.
But here again Huygens played a prominent part by first applying the
coiled balance-spring for regulating watches and marine clocks. The idea
of applying a spring to the balance-wheel was not original with Huygens,
however, as it had been first conceived by Robert Hooke; but Huygens's
application made practical Hooke's idea. In England the importance of
securing accurate watches or marine clocks was so fully appreciated that
a reward of L20,000 sterling was offered by Parliament as a stimulus
to the inventor of such a time-piece. The immediate incentive for
this offer was the obvious fact that with such an instrument the
determination of the longitude of places would be much simplified.
Encouraged by these offers, a certain carpenter named Harrison turned
his attention to the subject of watch-making, and, after many years of
labor, in 1758 produced a spring time-keeper which, during a sea-voyage
occupying one hundred and sixty-one days, vari