Roemer’s name is now best known by his discovery of the finite velocity of light. Most of his contemporaries doubted the reality of this discovery, chiefly because the eclipses of the three outer satellites of Jupiter did not show similar irregularities to those exhibited by the first satellite. This is not surprising, as the mutual attraction of the satellites makes their motions far more complicated than astronomers imagined before the development of the theory of gravitation, and it should perhaps be chiefly ascribed to chance that Roemer brought forward his theory, which Bradley’s discovery of the aberration of light about fifty years later proved to be a true one. Roemer’s ingenuity has, however, appeared very prominently in the important improvements which he carried out in the construction of astronomical instruments. The large “armil-lary spheres,” first constructed by the astronomers of Alexandria and also used by Tycho Brahe, had been superseded by the meridian or mural quadrant for measuring meridian zenith distances, and by the sextant for measuring distances between stars in order to find their difference of right ascension by solving the spherical triangle between the pole and the two stars. Both these instruments were introduced by Tycho Brahe. Roemer, however, saw that Tycho’s idea of making the rotating earth itself an astronomical instrument by observing the transits of stars across the meridian could be carried out better by fixing a telescope at right angles to a horizontal axis placed exactly east and west, so that the telescope could only move in the meridian. The first transit instrument was constructed in 1689 and erected in Roemer’s house in 1690. In the same year he erected in the university observatory an instrument with altitude and azimuth circles (for observing equal altitudes on both sides of the meridian) and an equatorial instrument. In 1704 he constructed a private observatory at Vridlösemagle, a few miles west of Copenhagen, and mounted a meridian circle (the transit instrument and vertical circle combined) and a transit instrument moving in the prime vertical. Roemer may thus be considered the inventor of nearly all our modern instruments of precision, and it is much to be regretted that his ideas were not adopted by astronomers until about a century later. All the results of his observations were destroyed in a great conflagration in 1728, except three days’ work, which has been discussed by Galle (O. Roemeri triduum οbservatiοnum astroenoemiearum a. 1706 institutarurn, Berlin, 1845). His disciple Horrebow has left us a very detailed description of Roemer’s instruments and ideas in his Opera mathematico-physiea (3 vols., Copenhagen, 1740-41). Grant’s History of Physical Astronomy (London, 1852) gives a very correct account of Roemer and his inventions. About his life see also an article by Philipsen in Nordisk Universitets Tidsskrift, vol. v., 1860.