Johannes Hevelius
observing with one of his telescopes. (Source:
Selenographia, 1647) The telescope was one of the central instruments of what has
been called the Scientific Revolution of the seventeenthcentury. It revealed hitherto unsuspected phenomena in the
heavens and had a profound influence on the controversy betweenfollowers 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 seethings that the great Aristotle had not dreamed of. It therefore
helped shift authority in the observation of nature from men toinstruments. In short, it was the prototype of modern scientific
instruments. But the telescope was not the invention ofscientists; rather, it was the product of craftsmen. For that
reason, much of its origin is inaccessible to us since craftsmenwere by and large illiterate and therefore historically often
invisible.
Although the magnifying and diminishing properties of convex and
concave transparent objects was known in Antiquity, lenses as weknow them were introduced in the West
They may have developed independently in China.
at the end of the thirteenth century. Glass of reasonable
quality had become relatively cheap and in the majorglass-making centers of Venice and Florence techniques for
grinding and polishing glass had reached a high state ofdevelopment. Now one of the perennial problems faced by aging
scholars could be solved. With age, the eye progressively losesits 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 ata 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 wornin a frame--spectacles. Because these little disks were shaped
like lentils, they became known as "lentils of glass," or (fromthe Latin)
lenses . The earliest illustrations of
spectacles date from about 1350, and spectacles soon came to besymbols of learning.
The Spectacle Vendor by
Johannes Stradanus, engraved by Johannes Collaert,1582 These spectacles were, then, reading glasses. When one had
trouble reading, one went to a spectacle-maker's shop or apeddler of spectacles (see
and
) and found a suitable pair by trial and
error. They were, by and large, glasses for the old. spectaclesfor the young, concave lenses
Note that the word lens was used only to denote convex lenses
until the end of the seventeenth century.
that correct the refractive error known as
myopia ,
were first made (again in Italy) in the middle of the fifteenthcentury. So by about 1450 the ingredients for making a telescope
were there. The telescopic effect can be achieved by severalcombinations of concave and convex mirrors and lenses. Why was
the telescope not invented in the fifteenth century? There is nogood answer to this question, except perhaps that lenses and
mirrors of the appropriate strengths were not available untillater.
