HISTOKY OF PHYSICAL ASTBONOMY.

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Olaus Roemer -was born at Copenhagen on the 25th of September,1644. When Picard proceeded to Denmark in 1671, for the purpose ofdetermining the exact position of Uranibourg, the scene of Tycho Brahe ’slabours, he found Roemer engaged in studying mathematics and astronomyunder Erasmus Bartholinus . Having discovered that the young man waspossessed of no ordinary talents, the French astronomer employed him asan assistant in his astronomical observations, and, at their conclusion in1672, he brought him along with him into France . In 1675 Roemercommunicated to the Academy of Sciences of Paris a memoir, in whichhe announced his discovery of the gradual propagation of light. Thisimportant communication obtained for him a seat in the Academy . Hehad also apartments assigned to him at the Royal Observatory, where hecontinued, during his residence in France , to^make occasional observationsof the celestial bodies. In 1681 he returned to Denmark , having beenappointed Royal Professor of Mathematics in the University of Copen-hagen. The great reputation which he had already acquired in France ,pointed him out as a person eminently qualified to undertake the dutiesattached to the observatory of that city. The peculiar construction ofthis edifice has already been briefly alluded to. Roemer did not fail soonto perceive that it could not be advantageously employed for the pur-poses of astronomical observation. In consequence of the instrumentsbeing placed at the very top of the building, they were constantly liable tobe disturbed in their positions by the violence of tempestuous winds. Itwas also found to be injurious to the health of the observer to prosecutehis labours in so exposed a situation. Roemer, therefore, resolved toobtain the erection of a more suitable observatory in some open district ofthe country; but having been unable to realise his views so soon as hedesired, he was induced in the meantime to convert his private dwelling-house into a temporary observatory.

The most remarkable novelty of the Observatorium Domesticum of Roe-mer was the Transit Instrument of which that distinguished astronomerwas the inventor*. It was placed so as to make observations with itat a window. In consequence of the interposition of the neighbouringhouses, it was impossible to command the view of a very large arc ofthe meridian. The range of the instrument extended from 20° southdeclination to 40° north declination. The axis, which was of iron, wasfive feet long, and an inch and a half thick. Its extremities restedupon iron supports inserted in the sides of the window, and the direc-tion of its position was east and west, so as to allow a telescope attachedto it to revolve freely in the plane of the meridian. The telescope wasplaced near one of the extremities of the axis. In order to obviatethe effects of flexure, the tube of the telescope was composed of twocones attached to each other at their bases. At the focus there wereinserted three horrizontal and ten vertical threads, but of the latter onlythree were used in practice. The illumination of the field of view waseffected by means of a lantern placed above the centre of the telescopewhich threw its light upon a perforated concave speculum inserted a littlebehind the object-glass, the speculum again reflecting the light alongthe interior of the tube. In order to admit the passage of the light tothe speculum, the upper portion of the tube of the telescope near theobject-glass was altogether removed. Towards the other end of the axiswas a projecting arm, carrying at its extremity a microscope by means of* Horrebow’s Basis Astronomies, p. 48, 4to. Hafnise, 1735.