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THE date of the foundation of the Observatory of Leiden University has long been uncertain. Lalande in his Astronomie says: 'En 1774 jen'y (i.e. at Leiden) vis ni Astronome ni instrumens que 1'on puisse citer'. The observatory did exist, however, at that date, and was certainly visible easily enough, but whether it deserved to be quoted in an enumeration of aétive observatories may be doubtful. Siegenbeek in the second part of his History of the University of Leiden, published in 18 3 2, gives the year 1782 as that of the foundation of the observatory. This date, however, is not of any importance in its history. The circumstance which probably gave rise to Siegenbeek's mistake is that in that year a certain Mr. van de Wal left his large telescope (which is still preserved and adorns the entrance hall of the present observatory, see fig. 4), as well as some other instruments, to the university. Kaiser, in an oration delivered at the opening of the renovated observatory in 1838, erroneously gave as the date of the foundation 1690, which is the date of the enlargement of the old observatory under the direftorate of de Volder. Later, on the occasion of the foundation of the new observatory, Kaiser searched the old archives very thoroughly, and most of the details given in the following short history have been borrowed from his published account in Vol. I of the Annals, or from his manuscript notes, which are still preserved at the observatory. A renewed search, for which I am much indebted to Mrs. Idenburg, the able secretary of the historical commission of the university, has brought to light some further interesting points that were not known to Kaiser.

The history of the old observatory on the roof of the university building, which existed from 1633 to 1861, is divided by Kaiser into four periods, viz: 1633—1680, 1680—1817, 1817—1838, 1838-186 1. In 1861 the new observatory, founded by Kaiser, was built on the present site, and with that date its career as an institution for scientific research may be reckoned to begin. The old observatory, though at some periods its equipment was decidedly modern for its time, never was considered as having any other funftion than being an aid to the teaching in the university, and populardemonstration.Thisexplainsthedisappointmentfeltbymany of the promotors of the plan for the new observatory when it was again annexed to the university, instead of being made, as they had hoped, an independent institution, like the meteorological institute.