LEIDEN OBSERVATORY 1633-1933















THE
UNIVERSITY OBSERVATORY AT LEIDEN
1633-1933



JACOBUS GOLIUS, the founder of the Observatory, front the oüpamtmgintheSenateroomofthelInh>ersity at Leiden (copy hy H. van der Mij, about 1740, of a seventeenth century original)



SHORT HISTORY OF THE
OBSERVATORY
OF THE UNIVERSITY AT LEIDEN 1633-1933
by
W.DE SITTER
PUBLISHED ON T THE GELEB RAT I O N OF T OF THE FOUNDATION BY JOH. ENSCHEDÉ E
HE OCCASION OF HE JOOTH ANNIVERSARY OF THE OBSERVATORY N ZONEN / HAARLEM







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.



The founder of the Leiden Observatory is Jacob Gooi, or Golhis, who in 1632 bought from the widow of Willebrord Snellius the great quadrant by Bleau, that had belonged to her husband, and persuaded the Curators of the University to take over this instrument and to build an observatory for it. The resolution of the curators to this effect was taken in their meeting of August 9, 1632, the plans and estimate of the architect Jan Jacobsz van Banchem were approved on November 15, the contract was given out on
Fig. 1. The Quadrant by Bleau, hoving belonged to Snellius, which gave rise to the foundation of the Observatory
December 2 for a sum of / 2 100. When this first building was completed does not appear. It consisted of a re&angular tower erefted over the roof of the western aisle of the university building (the former convent of the White Nuns) and was crowned by a flat roof or terrace of 19 by 15 feet. On August 9, 163 3, it was resolved to add another floor below the flat, and to allow Golius to use there two of the four globes that were kept at the library. These probably are the globes by Bleau, a terrestrial globe from 1599, and a celestial one from 1603, that are both still in existence, and were 'given on loan to the Museum for the History of Science at



WILLEBRORDUS* SNHJJUS PROFESSOR IHATHE^ro.S'
WILLEBRORD SNEL VAN ROYEN from the engraving by Jan van de Velde in Meursius' Athenae Batavae. 1625



Leiden in 1931. Finally, on November 14, 1633, the curators decided at Golius' request to build on the flat a small oftogonal turret or little house, and instruöed the builder Pieter Jansz. Banheyning, who had also built the flat, to submit plans. These were approved on May 24, 1634, the cost being ƒ 550. It appears, contrary to what Kaiser says in his history (Annalen der Sternwarte in Leiden, Vol. I, p. v), that this turret was not rotatable, but had 14 trap-doors, or shutters, permitting observations in different direftions. This completed the observatory building in its first state. No important changes appear to have been made either under the direftorate of Golius (163 3-1667), or of his successor Melder (1668-1681). A view of the observatory as it was in this first period 163 3 —1689 is given in fig. 2, which is a reprodu&ion of an engraving on the margin of Hagen's great map of Leiden of 1670. It is the same from which Kaiser's drawing fig. 1 on Plate I of Vol. I of the Annals was made. It gives a view of the university building from the east, and the observatory is seen projecting over the roof of the eastern aisle. The clock tower at the north end of the eastern aisle was built in 1670, and has nothing to do with the observatory. Kaiser's fig. 2 gives a view from the south on which the position of the observatory relatively to the building is clearly seen.
Such then was the beginning of practical astronomy at Leiden. Before Golius only Rudolph Snellius and his famous son Willebrord, Golius' teacher, had occasionally leftured on astronomy, but there were no facilities for making observations. The important ^work by Snellius, his triangulation and astronomical determinations of position, were made with instruments that were his private property, or borrowed from the Stadtholder Prince Maurice. The great Scaliger, to whom astronomy is so largely indebted for his introduftion of the Julian Day, was a professor at Leiden, but he can hardly be considered an astronomer. As it is, the Leiden Observatory is the first observatory ever connected with a public body such as a university. The only still existing observatory that is actually older is the Vatican Observatory, which was founded by Pope Gregory XIII (the reformer of the calendar) and thus dates from the sixteenth century. There had been observatories before, and famous ones, like Tycho Brahe's, and that of the Landgrave Wilhelm of Hessen, where Willebrord Snellius acquired his astronomical education, but they were private enterprises, and



Fig. 2. The University building with the Observatory seen front the east, from map of Leiden, drawn by J. de Jonge in 1670, engraved by Hagen



disappeared with the death of their owners. The observatory of Hevelius in Dantzig, founded in 16 41, was also a private one. Next in time come the observatory of the university of Utrecht in 1642, that on the Round Tower in Copenhagen in 1656, the Observatory in Paris, founded in 1667 by the Academy of Sciences, and the Royal Observatory at Greenwich in 1675. These appear to complete the record of the seventeenth century. Astronomy was in those times hardly considered as an independent science, Golius himself was professor of mathematics. The first professor at Leiden in whose tide astronomy was expressly mentioned was 's Gravesande, appointed in 1717, and the first professorate in astronomy alone was Kaiser's in 1840.
Golius must have been a very remarkable man. He was born in 1596. His father, Dirk Gooi, then lived at the Hague. He belonged, however, to an old Leiden family, had lived in Leiden during the siege and been a staunch supporter of the party of Burgomaster van der Werf and Janus Douza against those who wished to surrender, and for many years used to come back to Leiden on the third of Oftober to celebrate the reliëf. The young Jacob was inscribed as a student at Leiden in 1612. He studied medicine under Vorstius and mathematics and astronomy under Snellius, devoting special attention to opties, and took his doctors degree in 1616 on a dissertation 'de natura visus'. He then went back to his father's country house at Naaldwijk, devoting himself to the study of the Greek mathematicians, especially Apollonius. This made him realise the defeélive state of the available editions of the Greek authors, and kindled in him a desire to study them in Arabic translations. Accordingly in 1618 he went back to Leiden to study Arabic, the language of science at that time, under the famous Erpenius. Evidendy in this study he found the fulfilment of his innermost aspirations, and he soon became the enthusiastic collaborator and intimate friend of Erpenius, who was only twelve years his senior. In 1622 Golius was attached as 'engineer' to a diplomatic mission to the Sultan of Morocco, with the special instruction to study the possibilities of the port of Air, or Eyir (the present Agadir), where the States desired to found a fortified settlement or 'factory'. He spent most of his time in Safi in intercourse with Arabian scholars, and came home with a rich harvest of Arabian books and manuscripts. When



Erpenius died a few months after his return, a viftim to the epidemie of plague, Golius was appointed his successor, but at once secured leave for a voyage to the near East —in the capacity of 'Chancellor' of the Dutch Consulate at Aleppo. During this journey, which lasted nearly four years, he visited most of the important towns in Syria and Mesopotamia, as well as Constantinople, and not only gathered more treasures of Arabian literature, learned Persian and Turkish, and inctdentally earned great repute amongst the Turks as a doftor, but also made astronomical determinations of the positions of many towns and improved the map of the Turkish empire. After his return in 1629 he was also made a professor of mathematics, as successor to his former teacher Willebrord Snellius, who had died in 1626. His inaugural oration had for its subjeft the desirability of combining the studies of mathematics and Arabic!
For the suitability of this combination Golius' aftivity afforded excellent evidence. During nearly forty years he leftured every morning at nine on some subjeft of mathematics, opties or astronomy, and every afternoon at one on Arabian grammar, literature or history. The subjeft of the nine o'clock leftures was seldom the same for two years in succession. Sometimes it was on general mathematics and on the elements of Euclid, sometimes on spherical trigonometry after the Arabian text of Menelaus, in other years on opties or on astronomy —after 1663 with the addition of'hypothesin terrae motae'. Towards the end of his life such subjefts as 'on the general foundations of mathematics' or 'on the method of mathematical proof' begin to appear, alternating, however, with more elementary and practical leftures.
One of his acquisitions on his eastern travels of which Golius was most proud was a very complete Arabian manuscript of Apollonius' Conic Seftions, the defeftiveness of the extant Greek text of which had been his first inducement to learn Arabic, as we have seen, and he planned to prepare a latin translation of it. Notwithstanding the importance of this projeft, and repeated urgings from Huygens and others to bring it to completion, it was never accomplished. Aftually the first kon translation of Apollonius was published by Halley in 171 o.
Golius was well known in the scientific world of his time, and maintained a regular correspondence with many physicists, mathematicians and astronomers amongst his contemporaries. Des-



cartes, during his stay at Leiden, was a frequent guest at his house, and probably attended his leftures.
Of the observations that Golius made in his new observatory not much is known. Neither is it known what instruménts he had, beyond the great quadrant of Snellius. This had a radius of 7 feet, and a circle divided by transversals in 2''. The instrument is still in very good condition, and is kept in the observatory. Snellius and Golius used it with sights. The telescope shown in the photograph (fig. 1) wasadded in 166o by Melder. The quadrant was mounted on a strong vertical wooden pole, and could be rotated in azimuth. The way in which it was attached to this vertical axis can still be seen on the instrument as it is now. In 1656 Golius caused a divided azimuth circle of twenty inches radius to be added to the vertical axis. Whether Golius had other instruménts, and whether he had a telescope, does not appear. It is known that Snellius had a telescope as early as 161 o, but there is no indication whether it was bought by Golius from his widow at the same time as the quadrant. From the beginning Golius commanded the services of an attendant, to open and close the shutters for him, and to keep the instruménts in order, which duties he faithfully performed for an annual salary of ƒ 34, later increased to ƒ 40. When this funftionary died in 1652 Golius proposed as his successor a certain Schouten, who had some office in the University Press. The curators refused, saying that the beadles might do the work. But they could never long resist a wish expressed by Golius, and in the following year Schouten was appointed.
In Pingré's 'Annales Célestes du dix-septième siècle' several observations of cornets and of eclipses made by Golius and Kechel between 1630 and 1642 are mentioned, as well as observations of eclipses and two positions of the planet Mercury by an 'anonymus' at Leiden between 1640 and 1659. Pingré quotesthe observations from a MS of Boulliau, where the name of the observer is not given, but there is no reason why it should not be Golius or Kechel. In 1649 the anonymus had a clock showing seconds, in 1652 he used a clepshydra. Of course the clock of 1649 cannot have been a pendulum clock. Boulliau in 1652 derived the difference of longitude between Leiden and Paris from observations communicated to him from Leiden (probably a lunareclipse)and finds 81/s minutes. The true value is 8 minutes and 36 seconds. An error of not more



Fig. x. The University building with the Observatory seen from the west, from „Les délkes de Leyde", ijh (Historical Museum, Leiden University)
than a quarter of a minute must be considered as very small for that time.
Samuel Carolus Kechel was a native of Prague, born in 1611, and inscribed as a student at Leiden in 1632. He evidently afted as a volunteer assistant in the observatory. In 1666, however, he was granted an annual salary of/2 00 'in recognition of his faithful services during 3 2 years'. When Golius died in 1667, Kechel asked to be made his successor, but was not appointed. He was, ho wever, given permission to continue his observations in the turret on the university building, and his salary was raised to ƒ400. He died in 1668.



Golius' successor as professor of mathematics was Christiaan Melder, a physician from Dordrecht. He was also made a professor of the art of fortification, which office he evidently fulfilled with much enthusiasm. In 1672, the year of the French invasion, he commanded a company of students. In the same year the use of a piece of land was given him for practical exercises in military engineering. Of his astronomical activities not much is known, except that he added a telescope to the great quadrant. He leftured on military architecture, on the use of the globes, on Descartes' dioptrica, and on mathematical subje&s, and died in 1682.
The professorate of mathematics was then given to Burchardus de Volder (born 1643), who since 1670 had been professor of philosophy. De Volder, like Melder, was an ardent Cartesian, which led to difficulties with the curators, who insisted on the peripatetic philosophy being taught exclusively. He appears, however, to have continued his teaching according to his own conviftions, though the name of Descartes is not expressly mentioned in the 'series leftionum'. But leftures 'on metaphysical controversies' can hardly have been given by him without trespassing on the forbidden ground. De Volder was an eminent physicist. Huygens in his will had appointed him, with Fullenius of Franeker, as the editor of his posthumous works, of which task they acquitted themselves honourablyin 1703.
As a teacher of physics de Volder did pioneer work in founding the first physical laboratory, for which considerable sums of money were granted him for the purchase of instruménts, and where he demonstrated experiments for the students every Monday at 11 o'clock.
The observatory was considerably enlarged under his directorate. In 1682 he acquired a brass sextant, made by Metz of Amsterdam for ƒ 1050. The greater part of this instrument is still conserved in the observatory. It has a radius of 4 feet, and is divided by transversals in minutes. It is remarkable for having a screw attached to the arm for fine motion. In order to house this instrument a second turret was added to the south of Golius' turret, and the old one was rebuilt. The two turrets were joined by a platform resting on pillars. Both turrets had rotating roofs, and a shutter, which, however, did not allow observations at the zenith. The observatory in this second state, which lasted from 1689 to 1817,



Fig. 4. View in the entrance hall of the observatory with the telescope presented by Van de Wal in 1782, and the marble bust of Kaiser
is shown in fig. 1, taken from a sort of guidebook of Leiden, published by the bookseller Pieter van der Aa in 1712 under the title of 'Les délices de Leyde'. The view is taken from the botanical garden which is situated to the west of the university building.
In 1705 de Volder retired from office, and handed over to the curators two catalogues of instruménts, one of the physical laboratory and one of the observatory. The latter contains, besides the great quadrant and the sextant, two other quadrants, a telescope of twelve feet, two objectives, one of 18 feet and one of 5 o feet (focal length) and 'several oculars and smaller telescopes'. It also mentions 'two clocks showing seconds', but there is nothing to show that one of these is the pendulum clock made for Huygens by Thuret in Paris about 1670, as Kaiser supposes. When and how this clock



came to the observatory, where it still is an ornament of the director's room, and keeps excellent time, is unknown.
After de Volder had retired the curators took much trouble to find a worthy successor for him as diredor of the recently renovated and enlarged observatory. During the first two years Lotharius Zumbach de Coesfeld was permitted to use the observatory. He, however, wasappointed professor of mathematics in Kassei in 1708. The observatory then remained without a diredor for nine years, during which time the care of the instruménts was entrusted to Jan van Musschenbroek, a brother of the professor at Utrecht (later at Leiden). During this interregnum the university was presented with the 'sphaera movens', a very beautiful orrery construded by Steven Tracy, which in the course of time has cost great sums of money in repairs. It is now in the museum for the history of science. At last the curators thought they had found the long sought astronomer in Willem Jacob 's Gravesande, a young lawyer at the Hague (bom 1688), who had written on the air-pump and other physical and mathematical subjeds. He had been to England as secretary to a diplomatic mission and had there conversed with Newton and other great scientists. In 1717 he was appointed professor of astronomy and mathematics.
Although 's Gravesande was more a physicist than an astronomer, he acquitted himself honourably of his responsibility to the observatory entrusted to his care. He bought several new instruménts, as an objedive by Hartsoeker of 50 feet focal length, in 1727, a reflefting telescope by Hearne of 7 feet focal length, provided with a filar micrometer and said to be made under Bradley's supervision, in 1736, and a reversible meridian circle by Sisson, of 2^ feet focal length and a little over one inch aperture, having circles of o inches diameter, in 1740. Thus, when 's Gravesande gave the observatory over to his successor, it was well provided with modern instruménts. AU these instruménts are well preserved, and are now in the museum for the history of science. It does not appear that's Gravesande made any observations of importance. He lectured however on astronomy, including the motion of the planets according to Newton's theory, and instituted practical exercises for the students with the instruménts 'in Theatro astronomico'. The terminology is typical for the views held at the time regarding the fundion of the



Fig. 5. The University building with the Observatory from the east, from the lithograph by L. Springer, 1825
observatory. It was a place for public demonstration and teaching, not an institute for research. The first professor of astronomy at Leiden who tried to make observations with a view to purely scientific research was 's Gravesande's successor Lulofs, of whom more anon.
In 1724, after the death of de Volder's successor Senguerd, the physical laboratory was placed under 's Gravesande's direótion.



There he was in his real element. His name is still famous among the great representatives of experimental physics. Amongst his inventions were a heliostat with clockwork, by which he kept a beam of sunlight focussed on the same point for several hours, a new air-pump, and other contrivances. After his death his private colleóhon of physical instruménts was valued by Musschenbroek at ƒ3081.10 and bought from his heirs for this sum for the university.
's Gravesande died in 1742. His successor as professor astronomia? et matheseos and as diredor of the observatory was Johan Lulofs (born 1722). Lulofs was a true astronomer and an industrious observer. He observed Halley's cornet in 1759, the transits of Mercury in 1743 and 1753 and of Venus in 1761, made a new determination of the length of the seconds pendulum and published a very interesting investigation on secular changes of the level of the sea on the coast of Holland, which even to-day is well worth reading. Though he was well satisfied with the collection of instruménts brought together by his predecessor, he repeatedly asked for improvements in the building, which was antiquated and badly in need of repairs, but apparently without success. Also he complains bitterly about the hindrance in his observations caused by the general public, on such occasions as his observations of Halley's cornet, having no right to refuse admission to the observatory at any time. His oration as rector magnificus (1756) was 'on the impediments to the cultivation of astronomy in this country'. In 1768 he applied for a grant of money for a new instrument to enable him to observe the transit of Venus of 1769. The grant was allowed, but he died in November 1768.
His successor was Dionysius van de Wiinpersse, who, though 'professor philosophiaz, matheseos et astronomias', does not appear to have had any interest in or knowledge of astronomy. The astronomical teaching was probably left to Johannes Arent Fas, of whom nothing is known but that he had been appointed as lecturer in astronomy in 1763, under the French occupation, in 1811 at the age of 69, was made a 'professeür adjoint', and died in 1817. It was during the ebbtide under van de Wijnpersse (in 1774) that Lalande made the statement quoted above, that there was in Leiden neither an observatory nor an astronomer worth mentioning.
In the years 1785 and 1786 plans were under consideration for



new buildings for the university and the library. In the first plan (1785) the observatory was still proje&ed on the roof of the new university building. The plans or 1786 provided for a new observatory in a separate building on the ground level on a site to be offered for the purpose by the town of Leiden on one of the ramparts of the fortifications, near the Koepoort, just beyond the mill 'de Oranjeboom', i.e. about 800 meters east of the position of the observatory of 1861. These modern ideas seem to have originated with C. H. Damen, who since 1785 was professor of physics and mathematics. Nothing, however, came of the plans at that time, but it would appear that they were not entirely forgotten, but postponed for consideration at a more favourable time.
When Damen died in 1793, Pieter Nieuwland, author of a bombastic poem on the consteflation Orion, who enjoyed some reputation as a mathematician and had studied astronomy with von Zach at the new Ducal observatory near Gotha, was appointed 'professor Physices, Matheseos sublimioris, necnon Architecturae civilis, müitaris et hydraulica^, ut et astronomie'. Astronomy is included, as is explained, 'in order to relieve van de Wijnpersse of the cares of the observatory which became too pressing with his increasing age'. Great expectations were entertained of Nieuwland, which he had no opportunity either to fulfill or to disappoint, as he died in November 1794, only thirty years of age.
The curators then wished to secure the services of a good astronomer, worthy to direct the new observatory that they still had in



mind. While the negotiations with various persons were kept going, the salary of Fas was increased, and Speyert van der Eyk was appointed lefturer of mathematics and physics in 1796. He was promoted to be professor extraordinarius in 1797 and professor ordinarius in 1799-
The first to whom the curators offered the professorate of astronomy was the famous physicist van Swinden, who declined the offer. Then L'Huilier at Geneva, Oriani at Milan and Hindenburg at Leipzig were approached, and salaries up to ƒ 2600 were offered, but without success. Finally van Swinden was again asked at a salary of / 4000, but again declined. At last, in 1799, Jan Frederik van Beeck Calkoen was appointed as professor extraordinarius of mathematics, physics and astronomy at a salary of ƒ 1400. In 180 3 he was made professor ordinarius, and his salary was increased to ƒ 1600. On March 10, 1804, he held an inaugural oration 'on the latest discoveries in astronomy'. In 1805 he accepted a call to Utrecht.
Van Beeck Calkoen was a competent astronomer. He made observations of cornets with his students, and encouraged them to compute their orbits. Kaiser seems to imply that he left Leiden for Utrecht in disappointment over the slow progress of the plans for the new observatory. It seems more probable, however, that he was urged by more personal reasons, perhaps in conneftion with the state of his health, which was very weak. He died in 1811, 40 years of age. The plans for the new observatory on the site chosen by Damen were kept under consideration by the curators. In 1803 van Beeck Calkoen submitted a new plan, of which the cost was estimated at ƒ 30000. Although this exceeded by one half the sum that the curators had set as an upper limit, the plan was approved by them, and in September 1804 they authorized their president 'to do all that will be found necessary for the erection of the new observatory'. The times were, however, not propitious for such plans, and the curators are not to blame if nothing came of them in the end. In 1807, after the formidable explosion, which laid waste a large area in the middle of the city, plans were at once made to ereft a new university building on the space thus rendered available, and the erection of a new observatory on Damen's site, which is not far from the scène of the disaster, was again seriously considered. The building was to contain



FREDERIK KAISER from an engraving probably after the photograph by P. J. Kaiser, 1862



PROFESSOR H. G. VAN DE SANDE BAKHUYZEN, 1908



Fig. 9. The Observatory from the north-west, about 1900
Fig. 10. The Observatory from the south, about 1900
diately adjoining the western aisle of the university building). An excellent six-inch telescope by Merz-Fraunhofer was acquired and erefted in the north-west tower on the foundation made for it. This telescope was a comparatively large one at a time when the nineinch refraftor at Dorpat was the wonder of the world, commonly referred to as 'the giant telescope'. In addition Kaiser acquired a transit instrument and a universal instrument by Ertel, a clock by Mahler, and several other small instruménts, so that his observatory had become, as he expresses it, 'a charming little observatory, which might be the pride of an amateur, but was altogether too insignificant for a state institution'. The portable instruménts by Ertel could only be mounted in the south-eastern tower in 1845 when Kaiser at last, after many disagreeable altercations, got permission to demolish the 'grand telescope' which had been mounted in that tower since 1822. This was one of two practically useless reflecting telescopes made by a Frisian peasant of the name of Rienks, that had been bought by the King with the laudable intention of furthering the development of science. The other was destined by the King for the new observatory at Brussels, then being founded by Quételet. Owing to the separation of Belgium from Holland it



apparently never reached its destination, and what became of it is unknown.
In the new observatory Kaiser diligently made observations of cornets and minor planets, measurements of doublé stars and of diameters of planets, both with a filar micrometer and with Airy's doublé image micrometer, of which instrument he was an enthusiastic admirer. In the mean time he never lost sight of his ultimate aim, the foundation of an adequate national observatory in Holland, and more particularly in Leiden. By his teaching — he was a most inspiring speaker — and by his popular writings he created amongst all classes of the population an interest in astronomy such as had never before existed in this country. At last he was to earn the reward of his unremitting toil. In 1853 a commission was formed of prominent and influential men 'to promote the foundation of a new and adequate observatory at Leiden', under the chairmanship of Dr. J. Bake, professor of Latin at Leiden. This commission brought together a sum of nearly ƒ 26000 from contributions mosdy by private persons all over the country, as well as in the East and West Indies. The list of contributors makes interesting reading. Kaiser's native city Amsterdam heads the list with over ƒ 5000 from private persons, the 'Hollandsche Maatschappij' and "Teyler's Genootschap', both at Haarlem, contributed ƒ 1000 each, the professors of the university at Leiden gave more than ƒ 3600, and
Fig. 11. The West Tower, about 1900



the students ƒ 800. The most interesting contribution is that by Dr. R. Luther, directer of the Observatory at Dusseldorf, who gave 100 francs, being the Prix Lalande awarded him for his discovery of the planet Leucothea.
At first the gouvernment refused to make any move unless at least one half of the required funds were forthcoming from private contributions, but when Dr. G. Simons, a pupil of Moll at Utrecht, became Minister of the Interior, he at once included in the budget for 1857 the sum necessary for the erection of an observatory at Leiden. The budget was, however, voted down for political reasons. Fortunately Simons was succeeded by van Rappard, who was an enthusiastic admirer of Kaiser and a member of the commission already mentioned, and with the budget of 1858 the necessary funds were granted. The building proceeded regularly, and the commission could hold its last meeting on December 7, 1860, in the completed Observatory.
The observatory building, like all important buildings on the soft soil of this part of Holland, rests on piles, 1 o to 14 meters long, driven vertically into the ground to such a depth as to be permanently under the lowest level of the ground water. The wood then remains sound for an indefinite time. For the observatory building more than 1500 such piles were used, each of the piers of the meridian circle resting on more than 100 of them. This foundation has proved to be eminently stable.
For the site a piece of land just outside the town, adjoining the main road to the Hague, was first considered, but, fortunately for
astronomy, tne pnce asked for it was found too high. If the observatory had been built on that site, it would have been seriously hampered in its work
Fig, 12, The dome of the photographic telescope, about 1900



Fig. 13. The mam building from the north-west, 1933
ever since 1878, when the railroad from Leiden to Utrecht was built, passing within 100 meters of it. The present site was taken from the botanical garden of the university, enlarged by partly rilling up the old town moat which encloses it on two sides. It must be considered as an eminently favourable site, being situated in a very quiet corner far from any busy thoroughfares — in all respects decidedly superior to the site originally intended, and also to that envisaged in the plans of Damen and van Beeck Calkoen.
The observatory as it was in 1861 is shown in figures 7 and 8, which give views of the north facade from the north-east and north-west respectively. The middle part of the building contains on the ground floor office rooms, and on the first floor a large room, the roof of which could be opened, so as to enable observations with small telescopes to be made. To the east of the high central part of the building is the lecture room, to the west the meridian room, containing the 6-inch meridian circle by Pistor and Martins. The dire&or's residence is to the west of the meridian room, whilst



Fig. 14. The Building of the photographic department, 1933
symmetncal to it m the east are the living quarters tor the observators' and the mechanic and custos. Of the two domes the smaller or southern one contained —and still contains —the 6-inch Merz equatorial of 1838, the larger northern one contained a 7-inch refraftor by the same maker. Kaiser had asked for a o-inch, but the limited funds did not permit such an expenditure. The 7-inch was a very good instrument, as the excellent drawings of Mars made by Kaiser with its aid during the oppositions of 1862 and 1864 prove sufficiently. It was replaced by a telescope by Repsold (fig. 18) witha 1 o-inch lens by Clark in 1885, also a remarkably good instrument. The 7-inch was then dismounted and was never again used, until it was sent on loan to the Bosscha Observatory at Lembang in 1920.
A circumstance, which had favourably influenced the decision of parliament to grant the money for the new observatory, was that since 1853 Kaiser combined his professorship with the office of 'Verificator of nautical instruménts', in which capacity he had to test all compasses, sextants and other instruménts of the navy and



also to give time signals to the principal ports. This institution, which was created at Kaiser's own instigation, proved to be of great utility to the navy. It was housed in the observatory, in the room over the entrance hall. On Kaiser's death in 1872 hisson, Dr. P. J. Kaiser, succeeded him in this office, and in 187? two new rooms were added to the building for its accommodation on both sides of the entrance, as can be seen on fig. o. The same figure and fig. 1 o also show the new state of the two domes, which were rebuüt, the southern one in 1874—75, the northern one in 1888-89. The room over the hall was then used to accommodate the library.. Finally in 1883 the 'Verification' was removed to separate premises in another part of the city, and the rooms built in 187 3 were henceforth used as offices for the observatory.
In 1869 Kaiser acquired a transit instrument by Pistor and Martins, for which a small house, with two slits, one north-south and one east-west, was built in the garden to the south of the ledure room.
Fig. 15. The Observatory photographed from a military airplane in 1924



Fig. 16. General view of the Observatory buildings, 1933
In 1874 a Dutch expedition was sent to the ïsland of Réunion to observe the transit of Venus. After its return the hehometer by Merz of 85 mm aperture and about one meter focal length, which had been used by that expedition, was given to the Leiden Observatory. This gave rise to another addition to the buildings of the observatory, the West Tower (fig. 11), which was built in 1878. It has commonly been known under the name of the 'Heliometer Tower', though the heliometer was never mounted in it. At different epochs it has been used for the 6-inch telescope by Schröder and for various small instruménts. The heliometer itself was never used after its return from Réunion and is now in the museum for the history of science.
In 1803 a zenith telescope by Wanschaff was added to the instrumental equipment. The building to house it was erefted in 1898 to the east of the main building, in the same parallel as the meridian circle.
In the same year 1898 a small building was added north-east of the main building, containing a gas engine for the generation of electricity to charge an accumulator battery of 50 volts. After the reorganisation of 1919 the observatory was connefted to the city mains, and the building is now used as a smithy.



Staff of the Observatory, June 19 31
1. G. vanHerk 14. P. P. Bruna 26. L. J. F. van Leeuwen
2. P. Th. Oosterhoff 15. Prof. Dr. E. Hertz- 27. MissW.vanHeuzen
3. C. H. L. Sanders sprung 28. A. Luteyn
4. Ir. W. E. Kruyt- 16. J. M. Kriest 29. D. Gaykema bosch 17. M. D. Schepper 30. J. H. Kasten
5. J. Uitterdijk 18. B. C. Mekking 31. H. M. Swaak
6. Dr. C. H. Hins 19. L. Gaykema 32. Dr. J. Woltjer
7. A. J. Wesselink 20. J. C. Gaykema 33. J. Nicaise
8. Miss H. A. Kluyver 21. P. de Haan 3 4. G. Pels
9. G. P. Kuiper 22. J. E. Prins 35. F. de Haas
10. Dr. J. H. Oort 2 3. H. Zunderman 36. E. W. de Rooy
11. A. deSitter 24. Prof. Dr. W. de
12. Miss C. H. de Nie Sitter (Mr. C. }. Kooreman was
13. H. Kleibrink 25. J. E. Stol absentonmilitaryservice)



In 1887 the first international conference was held at Paris, where the undertaking of the Carte du Ciel was decided upon. Holland could not take part in this enterprise, but it gave the first impulse to the erection of a photographic telescope in the country. In 1878 a chair of astronomy had been founded in Groningen, to which
Fig. 17. The meridian circle
Professor J. C. Kapteyn was appointed. His desire was to have a photographic telescope, and the plans for it were developed, and even a site for the observatory was assigned. In the end, however, the photographic telescope was given to Leiden. The telescope was ordered from Gautier in 1896, the lenses being made by the brothers Henry. Its aperture is 340 mm, its focal length 524 cm.



It has a very strong English mounting (fig. 10). The dome was built in 1896—98, also containing a small dark room. Figure 12 gives a view of it as it was about 1900.
This was the latest addition to the instrumental equipment and the buildings until the reorganisation of 1 o 1 o.
Fig. 18. The 1 o-inch refractor
When Kaiser made the plans for his new observatory he had to decide on the policy which that institution was to follow. He realised very well that it would of necessity have to be what he called 'an observatory of the second order', that it would be out of the question to include in its aftivity the whole of astronomy, and that a choice must be made. Very dehberately he made his choice in



favour of fundamental astronomy. The meridian circle was to be the most perfect instrument of its kind that could be had at the time, provided with the most modern accessories and auxiliary apparatus. The reasons he gives for this choice are twofold. Firstly the high cost of large telescopes, which would be necessary for an adequate equipment for what then was called 'physical astronomy' (ï.e. the study of planetary surfaces, micrometric measures, etc), and secondly the greater need of investigations in fundamental astronomy, since large telescopes were then being erefted or planned at many places, 'even' in the United States of America. The decade 1860-1870, nearly coinciding with the period of
Fig. 19. The 1 yinch photographic rejraStor



Fig. 20.
The doublé camera
with movinfl plate-
holden
Kaiser's direftorship of the new observatory, saw the birth of the two powerful new methods, which were to revolutionise the science of astronomy, photography and spectroscopy. When Kaiser was, after his death in 1872, succeeded by his favourite pupil H. G. van de Sande Bakhuyzen, these new methods were yet in their beginning, and it was not easy to foresee the unparalleled development that they would undergo later. Also the inclusion of the new branches of astronomy in the programme of the observatory would have required the purchase of new and costly instruménts. Bakhuyzen accordingly decided to continue Kaiser's policy, and up to 1919 meridian astronomy remained the principal domam



Fig. 21. The first SchSt photometer
of activity of the observatory. In this domain it did excellent and fundamental work. I need only mention as one instance Bakhuyzen's discovery of the magnitude equation in meridian observations, first announced in 1877. Kaiser's own observations of Mars/ Kaiser's measures of planetary dia¬
meters, and of doublé stars, the participation of members of the staff in the expeditions to observe the transit of Venus of 1874 and the solar eclipses of 1001 and 190 5, are the only notable excursions into other regions pf the astronomical field. Even the new photographic telescope, installed in 1897, was not intended for astrophysical purposes but
Fig. 22. The azimuth instrument



Chief assistant: Assistants:
Voluntary assistants:
Chief of computing department: Computer highest grade: Computers lst class:
Computers 2nd class:
Non-estahlished computer: Typist:
Chief-instrumentmaker: Instrumentmaker and custos: Stoker and carpenter:
G. van Herk
Dr. P. Th. Oosterhoff
A. de Sitter
A. J. Wesselink
Ir. W. E. Kruytbosch P. P. Bruna
Miss H. A. Kluyver (secretary) J. Uitterdijk W. Chr. Martin J. G. Ferwerda
D. Gaykema
G. Pek
J. M. Kriest C. J. Kooreman J. E. Prins
E. W. de Rooy
H. M. Swaak
B. C. Mekking L. Gaykema
F. de Haas H. Kleibrink
Miss C. H. de Nie H. Zunderman J. H. Kasten P. de Haan
V
THE PRINCIPAL INSTRUMENTS OF THE LEIDEN OBSERVATORY
1. Meridian circle, by Pistor and Martins, 1861, overhauled by Repsold 1876-77, object glass repolished by Zeiss 1930. Aperture 16 cm, focal length 250 cm. Reversible. Two circles of 100 cm diameter, both divided to 5' and read by four microscopes. Provided in 1922 with impersonal micrometer, presented by Dr. Voute. (Fig. 17)
2. Photographic telescope, aperture 340 mm, focal length 524 cm and visual telescope, aperture 20 cm, focal length 524 cm, English mounting, by Gautier, 1897. (Fig. 19)
3. Visual telescope, lens by Clark, mounting by Repsold, 1885, aperture 266 mm, focal length 380 cm, German mounting. (Fig. 18)
4. Visual telescope by Merz, 1861, aperture 177 mm, focal length 280 cm, on loan to Bosscha Observatory, Lembang.



5. Visual telescope by Merz, 1838, aperture 15 cm, focal length 240 cm, German mounting.
6. Visual telescope by Schrö'der, aperture 160 mm, focal length 200 cm, English mounting, on loan to private observatory of Ir. W. E. Kruytbosch, Wassenaar.
7. Zenith telescope by Wanschaff, 1893, new objectglass by Zeiss 1929, aperture 90 mm, focal length 104 cm.
8. Azimuth instrument by Cooke, Troughton and Simms, 1930. Aperture 100 mm, focal length 100 cm. Broken horizontal telescope. Horizontal circle of 60 cm diameter, read by four microscopes, divided to 5' (Fig 22)
9. Doublé camera by Zeiss, 1924. Two identicalcameras, Zeiss Tessarlenses, 104 mm aperture, 52 cm focal length, guiding telescope of 100 mm aperture and 100 cm focal length. Motion of plateholders by eledric motors, so as to spread out the images into squares or rectangles of any size up to 1 x 1 mm. (Fig. 20)
10. Transit instrument by Pistor and Martins, 1869, aperture 75 mm, focal length 80 cm, provided with a registering micrometer bv Bamberg in 1897. 7 &
11. Transit instrument by Ertel, 1838, aperture 44 mm, focal length 51 cm, diameter of horizontal circle 30 cm.
12. Universal instrument by Pistor and Martins, 1858, aperture 30 mm, focal length 25 cm, diameter of circles 125 mm.
13. Universal instrument by Ertel, 1838, aperture 38 mm, focal length 37 cm, diameters of circles 19 and 24 cm.
14. Universal instrument by Sartorius, aperture 54 mm, focal length 65 cm, diameter of circles 35 cm, presented by Mr. Sanders.
15. Visual telescope by Zeiss, aperture 80 mm, focal length 141 cm, presented by Mr. Sanders.
16. Repsold measuring machine, 1876.
17. Repsold measuring machine, 1903.
18. Schlesinger measuring machine with long screw, made in our workshop 1931. 6 F
19. Schilt photometer, made in our workshop 1923, the first instrument of its kind. (Fig. 21)
20. Schilt photometer, made by van 't Hart at Rotterdam, 1931.
21. Stereocomparator for four plates, optical parts by Cooke, Troughton and Simms, mechanical parts made in our workshop, 1926.
22. Hartmann microphotometer, by Toepfer, 1929.
23. Sidereal clock Hohwü 17, 1860. An exceptionally good clock. the prinapal clock of the observatory, in a doubly closed niche in the entrance hall.
24. Sidereal clock Hohwü 46, 1897, in the meridian room.
25. Sidereal clock Knoblich, 1860, used to control the chronographs.
26. Sidereal clock Strasser und Rohde, 1899, a very good clock, used to control the system of electric dials throughout the buildings.
27. Mean time clock by Mahler, 1838.



UNDER CONSTRUCTION
28. Doublé photographic telescope for Johannesburg, by Sir Howard Grubb, Parsons & Co. Two identical Cooke tripiets, aperture 400 mm, focal length 229 cm, visual guiding telescope, aperture 200 mm, focal length 345 cm, English mounting. (Figs. 24 and 25)
29. Refleöing telescope, to be placed in the west tower. Mirror of aperture 45 cm, focal length (as Newtonian) about 225 cm, will be made by Mr. Kleibrink. Fork mounting under construction in our workshop, the heavy castings being made by Messrs Rademakers at Rotterdam. A mirror of 250 mm aperture and 215 cm focal distance, made by Mr. Kleibrink, will provisionally be used on this mounting, awaiting the completion of the larger mirror. (Fig. 26)
The Observatory in 1861, from the south-west



Fig. 7. The Observatoryin 1861 from the north-east
in four rooms ample accommodation for the instruménts, as well as a lefture room and a residence for the professor was to be annexed to it. These plans also were never realised.
After van Beeck Calkoen's departure the post remained vacant until 1811, when Cornelis Ekama was appointed, who remained in office till his death in 1826. In the mean time, in 1822, P. J. Uylenbroek was made lefturer of astronomy. In 1826 he was promoted to be professor extraordinarius. At the same time Kaiser was appointed to the newly created office of 'Observator' at the observatory. During this interval the plan for a new observatory was still in the mind of the curators, and was taken up again as soon as possible after the restauration of 1813. The country had regained its independence, but the university had lost its own. It was now a state institution, and by the statute of 181 5 all decisions about the equipment of the laboratories, or 'cabinets' as they were then called, were laid in the hands of the central gouvernment at the Hague. Evidently the funds for the foundation of a separate observatory according to the plans of Damen and van Beeck Calkoen were not available, and in 181 5 the curators proposed the renewal of the observatory on the roof of the university building as a temporary expediënt. Even so a useful, or at least usable, observatory might have been built at a moderate cost, as was done in 1837-1838 under Kaiser's guidance, but all that Ekama could think of, and what he obtained in 1817, was a cumbrous repetition of de Volder's observatory of 13 o years earlier. There were again two towers connefted by a platform, but not in the same position as de Volder's. Golius's tower was pulled down and replaced by a structure built on the top of the staircase, and another tower was added over the south end of the roof of the eastern aisle. This latter is well seen on fig. 5, which is a reproduétion of the procession of the



professors leaving the university building on the occasion of the celebration of the 2 5 oth anniversary of the university in 1825. The western tower was lower, and is concealed by the roof of the eastern aisle. A view from the west, also from 1825, is given by Kaiser, Plate I, fig. 5, of Vol. I of the Annals. This was the state of things in 1826, the year when Kaiser came to Leiden.
Frederik Kaiser was born at Amsterdam in 1808. His father, who had come to Holland from Nassau in the end of the eighteenth century, died when Kaiser was very young and he was educated by his uncle J. F. Keyzer, a merchant in Amsterdam, who was an enthusiastic amateur astronomer. Under his guidance the young nephew soon became proficient in the use of instruménts, as well as in astronomical computations. The uncle, however, also died when Kaiser was only 1 5 years of age. A few years afterwards, in 1826, Kaiser, on the recommendation of Moll, professor of astronomy at Utrecht (who himself had declined a call to Leiden as the successor of Ekama), was appointed 'Observator' at Leiden at a salary which released him from material cares and enabled him to pursue the study of his chosen science. Uylenbroek, who was at the same time appointed as Ekama's successor, was however diredor of the observatory, and there appear to have been constant causes of friction between the two men. Kaiser was a talented, enthusiastic, ambitious young man, but without any academie training, and possibly, as is often the case with self taught young men, a little apt to overestimate his own value and not always ready to show the proper respect due to the professorial dig-
nity of Uylenbroek, who was only eleven years his senior. Uylenbroek, on the other hand, according to all accounts, was a kind, honest, not very brilliant gentleman, who
Fig. 8. The Observatory in 1861 from the north-west



evidently had a strong feeling of the responsibility of his office, and not much faith in the capabilities and achievements of the unknown youngster who was thrust upon him without his being consulted in the matter. His difficult youth and the eleven years i82Ó-i837 of his early manhood thus spent under a constant strain, one might almost say oppression, certainly did not quench Kaiser's enthusiasm, but they left an indelible mark on his attitude of mind towards men and the world, and it is probably to their influence that must be ascribed a certain tendency, all too apparent in his published works, to overemphasise difficulties and worries, thereby often masking to the superficial reader the value of what he actually accomplished.
General acknowledgement of Kaiser's merits came as a resült of his work on Halley's cornet in 18 3 5. He had revised and combined the orbits computed by de Pontécoulant and by Rosenberger, and derived a new prediction of the date of perihelion passage, which afterwards proved to be correct within about an hour and a half. The use of the observatory being beset with too many difficulties for Kaiser, he observed the cornet, with a 41/1-inch telescope borrowed from Mr. Stoop of Amsterdam, from his own house, having removed sometiles from the roof. In recognition of this work he was madea doctor honoris causa in 1835.
Speyert van der Eyk having retired in 1835, Uylenbroek was made professor ordinarius of physics alone in 1837, and Kaiser was appointed lefturer of astronomy and director of the observatory. In 1840 he was promoted to be professor extraordinarius, and in 1845 to a full professorship. Funds for a new observatory were still not forthcoming, but the old structure on the roof of the university building was entirely renewed according to Kaiser's plans. The platform was taken down, leaving only a gangway between the two towers, as the only access to the south-eastem tower was through the north-western one. The two towers were provided with new rotatable roofs with proper shutters, and, what was even more important, in the north-western one a solid foundation of strong oak beams, resting on the walls öf the building and free from the floor, was constructed. Fig. 6 is a view of Kaiser's observatory, from a photograph taken in 1859, just before it was demolished, by Kaiser's son P. J. Kaiser. (The inclined chimneys seen between the two towers belong to the chemical and physical laboratory, which was then accommodated in a house built ïmme-



Fig. 2 3. The equatorial station near Equator, Kenya, occupied September 19 31 to March 1933, httkude -43*2, longitude 2h22mio"2, altitude 2900 meters. The observing hut is opened, showing the azimuth instrument, the zenith telescope and the chronograph. Seen from the north-east
for parallax and proper motion work. Praftically no work was, however, done with this instrument, until after 1919, when it was used for photometric investigations.
Both Kaiser and Bakhuyzen took a prominent part in organised international cooperation, which later developed to be one of the most characteristic features in the practice of astronomy, and began to come up just about the time of the foundation of Kaiser's observatory. Both were members of, and filled important offices in, the international geodetic association, and Bakhuyzen also took a prominent share in the international conferences of the Carte du Ciel.
During the reign of Kaiser and Bakhuyzen the scientific staff of the observatory had been gradually increased. In 1852 Kaiser succeeded in gètting the help of one 'Observator', of which post Oudemans was the first occupant. In 1908, when Bakhuyzen retired at the age limit, there were three observators and one 'assistant for the computarions' (whose tide was in 1911 changed to that of 'conservator'), besides a staff of 5 full time and x half-time compt*



Fig. 24. Model of the doublé 16-inch photographic telescope to be cretled at fohannesburg
ters, and three mechanics or instrumentmakers. In 1908, on Bakhuyzen's retirement, the directorate of the observatory was entrusted to his brother, the 'First Observator'
E. F. van de Sande Bakhuyzen, who in the following year was made professor extraordinarius. After his death in 1918 the directorate was again combined with the ordinary professorate of astronomy. The reorganisation of the observatory which took place in 1919, and the further development of the aftivities of the observatory under the present diredor, are too recent to be as yet treated as history, and only some of the most salient points can be mentioned here.
Fig. 2 5. The doublé 16-inch telescope for fohannesburg in the workshop of Sir Howard Grubb, Parsons and Co, August 1933



Fig. 26. Model ofthe 4 5-cm refleflor in construBion in the Observatory workshop, 1933
The most far-reaching innovation was the addition of the astrophysical department under the diredion of Professor Ejnar Hertzsprung, who has the title ofid 1 unct-d ireftor,
and is also a professor extraordinarius in the university. Intimately conneded with this are the relations with the Union Observatory at Johannesburg, which were initiated during a visit to Leiden of Dr. R. T. A. Innes in 1923, and will in the very near future be much extended by the aid of a grant given by the Rockefeller Foundation in 1930, supplemented by an increase of the annual budget from the Dutch government and annual contributions from private individuals and institutions in Holland. Of hardly less importance is the addition to the sphere of activities of statistical astronomy under Dr. J. H. Oort, and theoretical astrophysics and dynamical astronomy under Dr. J. Woltjer, both of whom have the title of Conservator and are ledurers in the university.
The most important additions to the instrumental equipment since 1919 are the doublé camera with moving plate holders by Zeiss (fig. 20), which was acquired in 1924, and the azimuth instrument by Cooke, Troughton and Simms (fig. 22), delivered in December 19 3 o, by which Dr. C. H. Hins and Mr. G. van Herk made observations for the determination of fundamental declinations at a station on the equator in Kenya from November 19 31 to March 1933 (fig. 23). Two new instruménts are under construdion, viz: adoublé 16-inch photographic telescope having triplet lenses of the Cooke type (figs. 24 and 2 5), for which the building, offered by the Gouvernment of the Union of South Africa, is now in course of eredion in the grounds of the Union Observatory at Johannesburg, and an 18-inch refleding telescope now being made in our own workshop, after the designs of our chief instrumentmaker, Mr. Zunderman (fig. 26).



The reorganisation of 191 o was necessarily accompanied by important changes in and additions to the buildings. The main building was enlarged by adding a itory on the top of the two rooms on both sides of the entrance, built in 1873 (see fig. 13). A new story was also added to the diredor's residence, and the eastern part of the thus increased house was annexed to the observatory for office rooms. Central heating and electric light were installed, and the whole inner arrangement of the old building was improved. Two new small domes were added on the main building to house the Zeiss doublé camera and the azimuth instrument. Office rooms for the new photographic department, rooms for measuring instruménts and dark rooms, were added adjoining the dome of the photographic telescope, and new residences for the chief instrumentmaker and the custos, as well as bachelor's quarters for the assistants, were built to the east of the photographic dome along the road leading to the observatory. All these alterations proceeded slowly, and it was not till 1924 that the reorganisation was so far completed that the new buildings could be officially opened by His Excellency the Minister of Education. Fig. 15 gives a general view of the establishment in the spring of 1924.
The enlargement and modernisation of the dwelling houses for members of the staff in the eastern wing of the main building were only completed in 1925.
In 19 31, after the granting of the gift by the Rockefeller Foundation and the concomitant increase of annual income by contributions from Holland, already mentioned above, the building for the photographic department was considerably enlarged by the addition of more office rooms, a store room for plates contaimng sufficiënt space for about 150000 plates and a small auxiliary workshop. The completed building is shown in fig. 14.
Finally in 1932 the domes of the west tower and the southern dome on the main building were renewed. A good general view of all the buildings is given in fig. 16, which is a reprodudion of a photograph taken from the roof of the zoological laboratory, situated diredly east of the observatory.
Thus the Leiden Observatory is well equipped to enter on its fourth century.



APPENDIX
PROFESSORS AND LECTURERS OF ASTRONOMY (AND OTHER SCIENCES) AT THE UNIVERSITY OF LEIDEN
Only' the nam es of those professors in mathematics, physics and allied sciences are given, who had some direct conneétion with the observatory or the teaching of astronomy.
1581 — 1613 Rudolph Snellius, professor of mathematics.
1593— 1609 Josephus Justus Scaliger, professor of Latin and ancient history
(in ven tor of the Julian Day). 1613—1626 Willebrordus Snellius van Royen, professor of mathematics. . 1629—1667 Jacobus Golius, professor of mathematics (since 1625 professor
of Arabic).
1668—1681 Christiaan Melder, professor of mathematics, dvü and military engineering.
1670—1705 Burchardus de Volder, professor of philosophy, physics and mathematics.
1717—1742 Willem Jacob 's Gravesande, professor of astronomy and mathematics, after 1724 also of physics.
1742—1768 Johan Lulofs, professor of astronomy and mathematics.
1763—1817 Johannes Arent Fas, lecturer (from 1811 professor) of mathematics, astronomy and navigation.
1769—1805 Dionysius van de Wijnpersse, professor of philosophy, mathematics and astronomy.
1785—1793 Christiaan Hendrik Damen, professor of mathematics, dvü and military engineering and hydrauhcs.
1793—1794 Pieter Nieuwland, professor of physics, mathematics, dvü and military engineering, hydrauhcs and astronomy.
1796—1835 Simon Speyert van der Eyk, lecturer of physics and mathematics (from 1797 professor extraordinarius, from 1799 professor ordinarius).
1799—1805 Jan Frederik van Beeck Calkoen, professor of natural philosophy and mathematics.
1811 — 1826 Cornelis Ekama, professor of mathematics and astronomy. 1822—1845 Pieter Joannes Uylenbroek, lecturer of astronomy and physics,
from 1826 professor extraordinarius, from 1837 professor
ordinarius.
1837—1872 Frederik Kaiser, lecturer of astronomy (from 1840 professor extraordinarius, from 1845 professor ordinarius).
1872—1908 Hendrik Gerard van de Sande Bakhuyzen, professor of astronomy.
1901 — 1910 Jan Hendrik Wilterdink, lecturer of astronomy.



1908— Willem de Sitter, professor of astronomy.
1909— 1918 Ernst Frederik van de Sande Bakhuyzen, professor extra¬
ordinarius of astronomy.
1919— Jan Woltjer, lefturer of astronomy.
1920— Ejnar Hertzsprung, professor extraordinarius of astronomy. 1930— Jan Hendrik Ooit, lefturer of astronomy.
II
SCIENTIFIC STAFF OF THE OBSERVATORY 1837-1919
DireSors: 1837-1872 F. Kaiser
1872-1908 H. G. van de Sande Bakhuyzen 1908-1918 E. F. van de Sande Bakhuyzen
Observatory. 1853—1856 J. A. C. Oudemans 1856-1859 M. Hoek
1859- 1869 N. M. Kam
1860— 1862 H. G. van de Sande Bakhuyzen (voluntary) 1863-1868 A. van Hennekeler
1869-1870 C. L. F. Kampf 1869-1871 W. Valentiner
/ J. de Jong ) (temporary, in conneftion
1874— 1875<H. Haga >with the transit of Venus
(W. Gleunsjof 1874) First observator: 1870-1871 E. Becker
1871-1875 W. Valentiner
1875- 1909 E. F. van de Sande Bakhuyzen 1910-1918 J. H. Wilterdink
Second observator: 1871 — 1875 E. F. van de Sande Bakhuyzen
1875-1878 J. C. Kapteyn
1878-1910 J. H. Wilterdink
1910-(1923)H. J. Zwiers Third observator: 1898-1906 A. Pannekoek
1907-1910 H. J. Zwiers
1910-1913 J. G. E. G. Voüte
1913 1917^' ^° ^>aron ^e ^os van Steenwijk "~ (J. Woltjer
1917-1920 J. Woltjer Conservator: 1911-(1924)J. Weeder
Assistants: 1875-1877 A. Eecen
1877-1883 T. J. Stieltjes
1883- 1884 A. Koolhoven
1884- 1888 W. Massink 1888-1911 J. Weeder 1890-1896 ƒ. Brandt



1890-1907 H. J. Zwiers 1892-1896 A. Pannekoek 1896 L. M. Klinkenberg.
III
SCIENTIFIC STAFF SINCE THE REORGANISATION IN 1919
Adjunct-direSors: 1919—
1920- 1922 Conservators: (1911) -1924
19201924-
Observators: (1907)-1923 19191924-1928 1928-
Chief assistant: 1927-1931 1932-
Assistants: 1920-1921
1921- 1923
1922- 1924 1924
1924- 1926
1925- 1928 1925-1927 1927-1928 1927-1933 192719281933-
E. Hertzsprung
J. C. Kapteyn
J. Weeder
J. Woltjer
J. H. Oort
H. J. Zwiers
C. H. Hins
W. H. van den Bos
H. van Gent
C. H. L. Sanders
G. van Herk W. J. Luyten
W. H. van den Bos J. Schilt
Miss J. M. Bruggeman
D. Brouwer
H. van Gent N. W. Doorn J. J. Raimond G. P. Kuiper
P. Th. Oosterhoff A. de Sitter A. I. Wesselink.
IV
STAFF OF THE OBSERVATORY, OCTOBER 1, 1933
Diretlor: Prof. Dr. W. de Sitter
Adjunct-diretlor: Prof. Dr. E. Hertzsprung
Conservators: Dr. J. Woltjer
Dr. J. H. Oort Observators: Dr. C. H. Hins
Dr. H. van Gent (at Johannesburg)