The idea would not let him rest until he had
accomplished his purpose. He cast and polished the speculum with
great labour; but just as he was about to finish it, the casting
broke! What was to be done? About one-fifth had broken away, but
still there remained a large piece, which he proceeded to grind
down to a proper diameter. His perseverance was rewarded by the
possession of a 3 1/2 inch speculum, which by his rare skill he
worked into a reflecting telescope of very good quality.
He was, however, so much annoyed by the treacherously brittle
nature of the speculum metal that he abandoned its use, and
betook himself to glass. He found that before he could make a
good achromatic telescope it was necessary that he should
calculate his curves from data depending upon the nature of the
glass. He accordingly proceeded to study the optical laws of
refraction, in which his knowledge of geometry and mathematics
greatly helped him. And in course of time, by his rare and
exquisite manipulative skill, he succeeded in constructing a
four-inch refractor, or achromatic telescope, of admirable
defining power.
The excellence of his first works became noised abroad.
Astronomical observers took an interest in him; and friends began
to gather round him, amongst others the late Professor Phillips
and the Rev. Vernon Harcourt, Dean of York. Cooke received an
order for a telescope like his own; then he received other
orders.
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