Fifteen years later, Ball, with a
telescope thirty-eight feet long, discovered the principal division in
the rings. Ten years still later, Cassini, with an instrument twenty
feet long and an object glass two and one-half inches in diameter,
rediscovered the division, which was named after him, rather than
after Ball, who had taken no pains to make widely known his discovery,
which, in the meantime, had been forgotten. Though we have no record,
there is no doubt that the lamented Horrocks and Crabtree, in England,
in 1639, with glasses no better than these, watched with exultant
emotions the first transit of Venus ever seen by human eyes.
In 1722, Bradley, with a telescope 2231/4 feet long, succeeded in
measuring the diameter of the same planet. Yet Grant assures us that,
in spite of all their difficulties, such was the industry of the
astronomers that when, at the commencement of this century, it became
possible to construct larger refracting telescopes, there was nothing
to be discovered that could have been discovered with the means at
their disposal.
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