The telescope (along with the microscope, another 17th century invention) demonstrated that ordinary observers could see things that the Greek philosophers had not dreamed of. It helped shift authority in the observation of nature from men to instruments.
News of the telescope's invention spread rapidly through Europe. By April 1609, three-powered spyglasses could be bought in spectacle-makers' shops on the Pont Neuf in Paris, and four months later there were several in Italy. They were made famous by an Italian professor and experimenter named Galileo Galilei in the summer of 1609 at the University of Padua near Venice.
While Galileo did not invent the telescope, he did design and build telescopes with increasingly higher magnifying power for his own use and to present to his patrons. He was a skilled instrument maker, and his telescopes were known for their high quality.
Galileo's first telescope was basically a tube containing two lenses. His first attempt was a three-power instrument; this was followed by one that magnified objects approximately nine times. He showed the latter device to the Venetian senate, hoping to impress them with its commercial and military potential.
Observations with Galileo's telescope strengthened the new idea that the Earth and the planets circled the Sun. It also revealed multitudes of stars in the Milky Way and elsewhere. One seemed to see not a fixed sphere of stars, but a universe of stars extending outward to some vast and unknown distance, perhaps to infinity.
Like the earlier Dutch versions, Galileo's refracting telescopes ("refractors") used lenses to bend, or refract, light. They featured a concave eyepiece lens and a convex objective lens. The telescope was fairly simple to make. Galileo, however, faced difficulties finding clear and hom*ogenous glass for his lenses. The glass was full of little bubbles and had a greenish tinge (caused by the presence of iron impurities). This was a problem that troubled telescope makers for centuries. It was also hard to shape the lenses perfectly. The images of stars were blurry, and surrounded by color haloes.
The limiting factor of these early refractors was their small field of view. Only part of the full Moon, for example, could be seen at one time. Galileo himself continued to improve his devices until they were over four feet long and could magnify up to thirty times.
