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HISTOBY OF PHYSICAL ASTRONOMY.

which the declinations of the celestial bodies were read upon a graduatedarc fixed opposite to it. The arc, which extended to 75°, was divided toevery ten minutes. In the common focus of the object-glass and eye-glass of the microscope there were inserted eleven parallel threads, bymeans of which the interval between two consecutive points of the gradu-ated arc was divided into ten equal parts. The interval between twothreads was consequently equal to a minute, and this again, according toHorrebow, the assistant of Roemer, might be subdivided by estimation so asnot to commit an error of more than 2|" or 3" in determining the declina-tion of a celestial body *. The pressure of the telescope and projecting armupon the axis of the instrument, was counterpoised by a weight tending topull it upwards at the centre. This contrivance was devised by Roemersome time after the instrument had been in use, in consequence of certainirregularities in the observations, which he could only account for by theflexure of the axis. The error of collimation of the telescope was ascer-tained by reversing the instrument. For this purpose a mark was care-fully observed in the open fields before the instrument was placed in itsposition. The adjustment of the instrument so as to make the telescoperevolve exactly in the plane of the meridian was effected by means of ob-servations of corresponding altitudes of the stars f. Horrebow statesthat the horizontality of the axis of rotation was carefully ascertained,but he does not mention by what means this object was effected.

Roemer continued to make observations with this instrument until asuitable observatory was erected for him in the country, when he obtainedanother of a similar kind but of a more perfect construction. Horrebow,his successor, states that in 1715, he prepared a suitable place for theoriginal instrument in the ancient observatory, and having transported itthither, made many thousand observations with it, which were registeredin fourteen folio volumes, when on the 21st of October, 1728, these,as well as all Roemer’s observations, and also the others which he himselfhad made, were destroyed by the fatal conflagration which consumed solarge a portion of the city on that occasion J.

* Roemer’s instruments were in general divided in the first instance to every ten mi-nutes, and afterwards subdivided to every minute by means of the microscope, the secondsbeing estimated as mentioned in the text. From the circumstance of eleven threadsbeing required to divide the space between two consecutive points on the instrument intoten equal parts, Delambre has hastily concluded that the threads performed the functionof a vernier, but this is obviously a mistake.

+ Delambre, who can see in Roemer only a person who carried out the ideas of Picard,endeavours, in every possible manner, to diminish the credit due to the Danish astro-nomer for the invention of this invaluable instrument; and, in order to attain his end, hehas not scrupled to throw out an insinuation no less at variance with truth than it is un-worthy of his high reputation as an historian of science. Thus, while trying to discoverthe reasons which induced Roemer to construct his instrument, he expresses himself in

tile following terms:—“.Peutetre Roemer n’a-t-il ete induit a la faire con-

struire que pour la raison unique que le mur de sa chambre etait oblique au meridienqu’il ne pouvaity placer un quart de cercle et pas meme la lunette murale de Picard.”(Hist. Ast. Mod., tome ii., p. 639.) Now so far from it being true that Roemer fellupon the construction of the transit telescope by mere accident, as Delambre has sug-gested in the foregoing passage, it appears from a letter which the Danish astronomerwrote to Leibnitz in 1700, that as early as the year 1675 be had discovered the peculiaradvantages of such an instrument, but was hitherto unable to obtain a suitable buildingwherein to place it. Thus he commences his description of the instrument in the fol-lowing terms:— “.De instrument), cui uni aptum sedifieium, jam per 25 annos

exoptavi, sed nunquam obtinere licuit.” (Miscellanea Beroliniensia, tome iii., p. 277.)

f Basis Astronomise, p. 55.