Portrait of the Scottish mathematician and astronomer James Gregory, 1638-1675. He graduated in mathematics at the University of Aberdeen and went on to study at Padua in Italy. Here he published a book in which he proved that a series, the sum of infinite terms, may have a finite sum. This result was able to break various paradoxes such as the twenty-one-century-old, also known as the paradox of Achilles and the Tortoise. Gregory also contributed to astronomy, suggesting that transits of Venus across the solar disk could be used to measure the distance between the Earth and the Sun. He proposed before Newton that telescopes could be made with mirrors instead of lenses.