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.



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 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 near by 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.



These spectacles were, then, reading glasses. When one had  trouble reading, one went to a spectacle-maker's shop or a  peddler of spectacles (see Figure 2 and Figure 3) and found a suitable pair by trial and  error. They were, by and large, glasses for the old. spectacles  for the young, concave lenses  that correct the refractive error known as myopia,  were first made (again in Italy) in the middle of the fifteenth  century. So by about 1450 the ingredients for making a telescope  were there. The telescopic effect can be achieved by several  combinations of concave and convex mirrors and lenses. Why was  the telescope not invented in the fifteenth century? There is no  good answer to this question, except perhaps that lenses and  mirrors of the appropriate strengths were not available until  later.



In the literature of white magic, so popular in the sixteenth  century, there are several tantalizing references to devices  that would allow one to see one's enemies or count coins from a  great distance. But these allusions were cast in obscure  language and were accompanied by fantastic claims; the  telescope, when it came, was a very humble and simple device. It  is possible that in the 1570s Leonard and Thomas Digges in  England actually made an instrument consisting of a convex lens  and a mirror, but if this proves to be the case, it was an  experimental setup that was never translated into a  mass-produced device.

Source: http://cnx.org/content/m11932/latest/