In 1658, Huyghens, using a telescope twenty-three feet long and two and one-third inches in diameter, with a power of 100, solved the mystery of Saturn's rings, which had resisted all of Galilei's efforts as well as his own with a shorter instrument, though he had discovered Titan, Saturn's largest moon, and fixed correctly its period of revolution at sixteen days. 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. So far as we now know, a good three-inch telescope, nay, a first-rate two inch one, will show far more than our great-grandfathers ever saw, or dreamed of seeing, with their refractors.