ROEMER, OLE (Latinized OLAus) (1644-1710), Danish observatory (Uranienburg on the island of Huen). In 1672 he accompanied Picard to Paris, where he remained nine years, occupied with observations at the new royal observatory and hydraulic works at Versailles and Marly. In 1675 he read a paper before the Academy on the suc-cessive propagation of light as revealed by a certain into Copenhagen, where he spent the remainder of his days as professor of astronomy, but his great ability and prac-tical talents were made use of in several other public employments. He died on 23d September 1710.

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 sirnilar irregularities to those exhibited by the first satellite. This is not surprising, as the mutual attpaction of the satellites makes their /notions 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 pro-minently 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large " armil-lary spheres," first constructed by the astronomers of Alexandria and also used by Tycho Brahe, had been superseded by the meridian or mural quadrant for measuring meridian zenith distances, and by the sextant for measuring distances between stars in order to find their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 hori-zontal axis placed exactly east and west, so that the telescope could only move in the meridian. The first transit instrument WaS con-structed 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 Vridliisemagle, a few miles west of Copenhagen, and mounted a meridian circle (the transit instrument and vertical circle combined) and a transit instrument n:oving 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 astro-nomers until about a century later. All the results of his observa-tions were destroyed in a great conflagration in 1728, except three days' work, which has been discussed by Calle (O. Roemeri triduum observationum astronomicarum a. 1706 instit2ttarum, Berlin, 1845). His disciple Horrebow has left us a very detailed description of Roemer's instruments and ideas in his Opera mathematico-physica (3 vols., Copenhagen, 1740-41). Grant's History of Physical Astro-nomy (London, 1852) gives a very correct account of Roemer and his inventions. About his life see also an article by Philipsen in Nordisk Universitets Tidsslcrift, vol. v., 1860.