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.
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