Investigations by scientists such as Tycho Brahe, Galileo, and Johannes Hevelius stretched the boundaries of the known universe into the heavens. The invention of the telescope in 1609 resulted in "detailed maps of the moon and elaborate astronomical diagrams that offered a glimpse of a world far beyond man’s reach." The seventeenth century also saw the invention of better magnifying lenses and the microscope, causing the Dutchman Constantijn Huygens to write: "we wander through a world of tiny creatures till now unknown, as if it were a newly discovered continent of our globe."
The last frontier was the core of the earth itself, not yet readily measured or seen by the human eye, but with properties that could be deduced by observing physical phenomena such as mountains, crevices, and tides. The concepts of inductive and deductive reasoning, proposed by the French philosopher René Descartes, helped make possible scientific propositions about unseen phenomena by such men as Athanasius Kircher whose Mundus Subterraneus (1664) was a study of the subterranean world.
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