But as soon as the principle by which distant objects could, apparently, be brought nearer the eye became known and its value recognized by philosophers, telescopes ceased to be regarded as toys, and underwent material improvements in the hands of such men as Galilei, and, later, even of Kepler himself, Cassini, Huyghens, and others.  Galilei’s first telescope magnified but three times, and his best not much above thirty times.  If I comprehend aright what has been written upon the subject, I am justified in saying that this little instrument in my hand, with an aperture of one inch and one-quarter, and a focus, with an astronomical eye-piece, of about ten inches, is a better magnifier than was Galilei’s best.  With it I can see the moons of Jupiter, some spots on the sun, the phases of Venus, the composition, in some places, of the Milky Way, the seas, the valleys, the mountains, and, when in bold relief upon the terminator, even some of the craters and cones of the moon.  Indeed, I am of opinion I can see even more than he could, for I can readily make out a considerable portion of the Great Nebula in Orion, some double stars, and enough of the Saturnian system to discern the disk of the planet and see that there is something attached to its sides.

For nearly one hundred and fifty years all refracting telescopes labored under one serious difficulty.  The images formed by them were more or less confused by rainbow tints, due to the bending, or refracting, by the object glass of the rays of light.  To overcome this obstacle to clear vision, and also to secure magnification, the focal lengths of the instruments were greatly extended.  Telescopes 38, 50, 78, 130, 160, 210, 400, and even 600 feet long were constructed.  I can, however, find nothing on record indicating that the object glasses of these enormously attenuated instruments ever exceeded in diameter two and one-half inches.  Yet, with unwieldy and ungainly telescopes, nearly always defining badly, wonders were accomplished by the painstaking and indomitable observers of the time.

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.