Someone from the Netherlands found a new use for eyeglass lenses early in Galileo’s time. What was it?

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Eyeglass lenses were used to form the first-ever telescopes. However, there is actually some controversy over who invented the first telescope and where and when they invented it.

The standard answer is that the inventor of the telescope was Hans Lipperhey. (See the link below.) Lipperhey, a lens maker, applied for a patent of the telescope to the States General in the Netherlands in 1608. Shortly afterward, another Dutchman, by the name of Jacob Metius, applied for a patent and claimed to be the true inventor of the telescope. Decades later, some evidence emerged that a Dutch lens maker named Sacharias Janssen may have actually invented the telescope first, in 1604. The claim was made by his son. To add to the controversy, Janssen's son also claimed that his father had constructed the telescope based on an Italian device dated to 1590. Metius and the Italians are usually marginalized in the discussion, which has sometimes been termed the "Lipperhey-Janssen debate."

We do know that Galileo was the first to productively use the telescope to challenge some of the prevailing perceptions of the movement of the heavenly bodies and to effectively disseminate his findings.

An impressive amount of historical research has been conducted on the origins of the telescope, the development of the device, the progress in the theoretical knowledge that accompanied that development, and the practical implications of that progress for astronomy in the seventeenth century. De Waard, Van Helden, and Zuidervaart are among the historians whose works one may want to consult if looking deeper into the matter.

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The first eyeglass lenses had been made in Italy in the 14th century, and several new types were developed in the 15th century. Dutch scientists developed lens that were superior for distance, and they combined them into a device that made far away things seem close. Inside a wooden tube, both convex and concave lens were used. The first such device was exhibited in 1608 in The Hague, in support of a patent application, but the patent was not then awarded. Galileo constructed his first telescope in June 1609, and by November of the same year, he had made two more, increasingly powerful ones. With them, he was able to observe the moon and planets, and he published about his observations in 1610.

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