On August 25, 1609, Galileo Galilei first demonstrated his new construction, the telescope, to Venetian lawmakers. Magnifying distant images at first by three times, then, soon after, by about eight or nine times, it quickly became a profitable sideline for Galileo, who sold his telescopes to merchants who found them useful both at sea and as items of trade. He published his initial telescopic astronomical observations in March of the next year.
Galileo did not exactly invent the device, but perfected it for astronomical observation. It was called a “Danish perspective glass” in the version that had inspired him, and the idea might have been ancient. It was at least not wholly new, as suggested in the great short story “The Eye of Allah” by Rudyard Kipling.