io6 



NA TURE 



\Pec. 6, 1877 



the memory of that great man that Hevel in Danzig 

 carried cut Tycho's ideas about his observatory, and 

 rejected all the improvements that had since originated, 

 amongst which was the application of the telescope to astro- 

 nomical observations. The obstinacy with which Hevel 

 refused to adopt this invention appears strange to us now, 

 but we must remember the great accuracy which was then 

 obtained by pinnules alone. Tycho had reduced the 

 probable error of astronomical observations from ten 

 minutes to one, and some of Hevel's observations have 

 been found to be affected by errors of less than half a 

 minute of arc, results which show that the old astro- 

 nomers were in possession of a skill in handling their 

 apparatus which has since been lost. It should also 

 be taken into account that the telescopes of Hevel's 

 day were generally of Dutch construction, and 

 Kepler's tube, with wires in the field to mark the 

 centre, was first brought into general use by Auzout 

 and Picard about the end of the century. . Of hardly 

 less importance was the application of the pendulum 

 to clocks, which from that time have been used as 

 astronomical instruments. They had in Tycho's 

 observatories been used merely to show what o'clock 

 it was when observations were made, but never to 

 determine differences of right ascension. 



With sufficiently good clocks it was possible to 

 determine the positions of the stars by observa- 

 tions in the meridian alone, and it was no doubt 

 Picard who first became aware of the immense 

 advantage of this. Consequently he solicited Go- 

 vernment for a large mural quadrant, but Cassini 

 was then called in from Italy, and no notice was 

 taken of the request made by Picard, who, unfor- 

 tunately for the practical astronomy of France, was 

 not thought much of by the court of Louis XIV., 

 his important, but modestly-conducted researches 

 being eclipsed by Cassini's brilliant discoveries. 

 Had Picard got the direction of the Royal Obser- 

 vatory in Paris he would have been able to make 

 further improvements in the construction of instru- 

 ments ; but with no sufficient means at hand, he 

 ascribed the partial failure of his attempts to the 

 small size of his instruments. A mural quadrant 

 like Tycho Brahe's, but furnished with a telescope, 

 was first fixed at the observatory when Picard 

 died.' Flamsteed. and Sharp adopted the methods 

 just as Picard left them and with all their draw- 

 backs. They used the quadrant both for right 

 ascensions and declinations. Theit observations 

 may perhaps be said to be twice as accurate as 

 Hevel's naked-eye observations. 



This was the state of practical astronomy when 

 Romer raised it to a height which was not sur- 

 passed before Bessel. Ole Romer was born in 

 Aarhus on September 25, 1644. Thence he came, 

 1662, to Copenhagen, where he studied mathe- 

 matics and astronomy under Erasmus Bartholin, 



whom he subsequently assisted. As has already been 

 pointed out, Tycho's observations continued to be 

 consulted by astronomers, and in 1671 Picard went 

 to Denmark to determine the difference between the 

 longitude of Uraniburg and Paris.' There he found 

 Romer occupied in revising Tycho's manuscripts, and he 

 secured his assistance in the observations on Hveen, and 

 when Picard returned to France he procured Romer a 

 place as assistant at the observatory of Paris. There his 

 talents did not fail to be appreciated, and he was soon 

 elected a member of the Academy. It was in Paris that 

 Romer discovered the gradual propagation of light from 



QVADRANS MVRALIS 



SIVE TICHONICUS. 



' This interesting irstru lent is represented in Fig. i. It was cast 

 wholly in bi ass and fi>ed with strong iron screws as exactly as possible 

 in the meridian in the south-western room of the ground-floor of Uraniburg. 

 Its radius was about six feet, and it could by means of transversals be read 

 off to ten seconds of arc It had in a hole in the south-western wall in the 

 centie a g>lt cylinder and two pinnules movable along the edee, which were 

 so constructed tbat the slit could be opened or closed more or less according 

 to the faintness or brightntss of the objects to be observed. Tycho Brahe 

 who, in contiadist notion to Ole Romer, was not only anxious about the 

 quality but also about the appearance of his instruments, had ornamented 

 the large empty s]^ ace of the quadrant with the splendid picture shown in 

 the plate. He is here depicted in his usual attire. At his feet is lying one 

 of his favourite huntir.g-dogs, more as a symbol of ingenuity than as a 

 symbol of nobility. Behind him are small poi traits of King Frederick and 

 Queen Sofie. This was painted by John of Antwerp and is more like him 

 than any other image, but the space contained also an architectonic picture 

 by Steenvinkel, somewhat reduced as if at a distance. In the upper Story 

 are reprcseiited some of his most celebrated instruments, in the middle story 

 the library mside with the large celestial globe ana his pupils occupied with 

 their studies, and in the cellar the chemicti laboratory. On the plate is seen 

 a young man observing through one ct the pinnules, anotttr is watching the 

 clocks, while a third is noting down their remarks at a table. Tj'cho Brahe's 

 image seems as if admonishing and instructing them in their work. 



Fig. I. 



observations of the eclipses of the first satellite of Jupiter. 

 His results, which were not very exact, were however, 

 contested by Cassini and most other authorities for a long 

 time after. Indeed, the theory of the moti 'n of the 

 satellites was at that time so little elaborated, t lat similar 

 conclusions might be questioned all the more a - they had 

 been deduced from observations of the firs satellite 

 alone. Romer shortly afterwards discovered the epi- 

 cycloid, and published a paper on the prope- form of 

 toothed wheels, and descriptions of a Jovila )ium and 

 Saturnilabium ; he afterwards invented differen: kinds of 



' On this occasion fire-signals were for the first time made use of for the 

 determination of longitude. A fire was lit on the top of the astr jnomica 

 tower in Copenhagen. There Picard eclipsed it at rsgular in-e <als, aad 

 the moment the light disappearai was noted by the observers oa Hvetn. 



