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A brief history of Galileo's telescope.

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.

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Source:  OpenStax, Galileo project. OpenStax CNX. Jul 07, 2004 Download for free at http://cnx.org/content/col10234/1.1
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