Galileo's telescope

The telescope was one of the central instruments of what has
been called the Scientific Revolution of the seventeenth
century. It revealed hitherto unsuspected phenomena in the
heavens and had a profound influence on the controversy between
followers of the traditional geocentric
astronomy and cosmology and those who favored the
heliocentric system of
Copernicus. It was the first extension of one of man's
senses, and demonstrated that ordinary observers could see
things that the great Aristotle had not dreamed of. It therefore
helped shift authority in the observation of nature from men to
instruments. In short, it was the prototype of modern scientific
instruments. But the telescope was not the invention of
scientists; rather, it was the product of craftsmen. For that
reason, much of its origin is inaccessible to us since craftsmen
were by and large illiterate and therefore historically often
invisible.



Galileo's telescope


Although the magnifying and diminishing properties of convex and
concave transparent objects was known in Antiquity, lenses as we
know them were introduced in the West
1
at the end of the thirteenth century. Glass of reasonable
quality had become relatively cheap and in the major
glass-making centers of Venice and Florence techniques for
grinding and polishing glass had reached a high state of
development. Now one of the perennial problems faced by aging
scholars could be solved. With age, the eye progressively loses
its power to accommodate, that is to change its focus from
faraway objects to nearby ones. This condition, known as
presbyopia, becomes noticeable for most people in
their forties, when they can no longer focus on letters held at
a comfortable distance from the eye. Magnifying glasses became
common in the thirteenth century, but these are cumbersome,
especially when one is writing. Craftsmen in Venice began making
small disks of glass, convex on both sides, that could be worn
in a frame--spectacles. Because these little disks were shaped
like lentils, they became known as "lentils of glass," or (from
the Latin) lenses. The earliest illustrations of
spectacles date from about 1350, and spectacles soon came to be
symbols of learning.