464

HISTORY OP PHYSICAL ASTRONOMY.

observations with it. The deviation of the optical axis of the telescopefrom a line perpendicular to the axis of rotation, or in other Vords, theerror of collimation, was ascertained by reversing the instrument. Asthis mode of verification was troublesome, he subsequently effected the sameobject by observing two distant marks in the horizon, diametrically oppositeto each other. The horizontality of the axis of rotation was ascertained bymeans of the plumb-line. The adjustment in the plane of the meridianwas effected by the aid of observations of Capella and # Lyrse, duringtheir passages above and beneath the pole.

The other instrument employed by Roemer in his Observatory was atelescope revolving in a plane perpendicular to the meridian. It was fur-nished with a system of vertical threads at its focus, similarly to the tele-scope of the meridian circle, being designed to be used in observing thepassage of the celestial bodies across the prime vertical*.

In order to exhibit an illustration of his mode of observing, Roemer drewup a list of the declinations and right ascensions of certain stars, deter-mined by him with the meridian circle on three consecutive nights in themonth of October, 1706. This small collection received from him, in

* It is to be regretted that Delambre, who was so eminently qualified to appreciate thevalue of the improvements in practical astronomy effected by Roemer, has allowed hismind to be so overruled by preconceived notions as to have been betrayed into a com-plete misrepresentation of the labours of that astronomer on the present occasion. Thus,in his description of the Observatorium Tusculaneum, the following extraordinary pas-sage occurs:—“ On y voit deux portes, quatre fenetres, un lit, une cheminee, quatrehorloges, et deux lunettes, l’une meridienne, I’autre, plus petite, qui tournait dans le pre-mier vertical. Mais ce qu’il y a de singulier c’est que de quatre fenetres, deux sont dansla direction de l’axe de rotation, et qu’ une seule tout au plus parait etre dans la directionde la lunette principale(Planche VIII.), et que la Planche Ill.destinee principalement afaire connaitre l’instrument, et qui nous montre Roemer occupe a observer, parait prouverVimpossibilite absolue de diriger la lunette au nord; et quand tout serait ouvert, et la lunettetournee au nord, on neconcevrait pas mieux comment l’observateurpourrait passer du cote del’oculaire.” (Hist. Ast. Mod . tom. ii., p. 654). It will be evident to any person who be-stows even a passing glance over the pages of Horrebow’s Basis Astronomies, that Delambre,in the foregoing passage, confounds the Observatorium Domesticum fitted up by Roemer ina corner of his house in the year 1690, with the more complete edifice erected by him in1704, which is denominated by Horrebow the Observatorium Tusculaneum. In fact,Plate III. refers to the former of these observatories, and to the original transit instru-ment (termed by Horrebow the Machina Domestica ), while, on the other hand, PlateVIII. is intended to represent the Tusculan Observatory with the Rota Meridiana andthe prime vertical telescope. It has been mentioned in the text that the observationswith the Rota Meridiana , instead of being made at a window barely opposite to thetelescope , as Delambre seems oddly enough to imagine, were in reality made through afissure in the building, situate exactly in the plane of the meridian; and that so far fromits being impossible to direct the telescope towards the north, the extent of the fissureallowed its being directed to any point in the meridian included between the northernand southern horizons. The following words of Horrebow are so explicit upon thispoint that it seems incredible that they should have escaped the notice of Delambre“ Ut autem tubo cum machina primaria M (Rota Meridiana) eircumvoluto totus meridi-anus ab horizonte australi per zenith usque ad horizontem borealem pateret, parieteseedificii cum tecto acuminato rimam continuam habebant quatuor pollices latam, .(Basis Astronomies , p. 141). The following remark of Delambre’s in reference to thesame subject is, perhaps, still more calculated to excite astonishment than anything thathas yet been cited:—“ Horrebow regrette qu'on n'eut pas la faculte d'observer les etoilescircumpolaires; ses planches paraissent demontrer cette impossibility et cependant lapolaire; la tete du Dragon, la Lyre , la tete et la queue du Cygne, deux etoiles de laqueue de la Grande Ourse, ont ete observees au dessous comme au dessus du pole.”(Hist. Ast. Mod., tom. ii., p. 654). The words in Italics, which are so inserted in theoriginal, with the evident intention of impressing their significance more strongly uponthe reader, afford a remarkable illustration of the facility with which a person may be