Light

Rasmus Bartholin, a member of the Bartholin family which played a dominant role in the University of Copenhagen for about a hundred and fifty years, was a professor of mathematics and medicine there. In 1669, he published his observation of a phenomenon. Studying the transition of a beam from air into a crystal of Icelandic spear, he discovered double refraction. Upon entering that crystal, light suffers not one deflection but two at once, the beam splits into two parts.

Whether the velocity of light is finite or infinite had been much debated through the centuries. Aristotle, Kepler, and Descartes opting for Infinity. Ole Romer, Bartholin's amanuensis, later his son-in-law, working at the recently established Royal Observatory in Paris, measured for the first time the velocity of light. Training a telescope on Io, Jupiter's innermost moon, Romer found, in 1676, that this satellite shows a peculiar variation in its motion around Jupiter, from which the value 214,300 kilometers per second for the light velocity could be deduced -- about two-thirds the modern value --. Romer's work provides example of the marvels revealed by the new 17th century instrument, the telescope, a culmination of the development of lenses that had begun in the late 13th century.