i 



undettaken at the Konigsberg Observatory. 429 



cupied in a series of observations of the circum-polar stars, in 

 order that the properties of the instrument and the reductions 

 employed in the observations might be determined. I thought 

 it right to let these observations precede, without mterrupting 

 the course of them by any others, since they must be very 

 numerous to ensure the desired certainty ; and no advantage 

 would have resulted from extending very widely the knowledge 

 of the true method of reduction. In August 1821, however, 

 these preparations, and others of which I shall hereafter speak, 

 were completed, and I possessed in Dr. Argelander an assist- 

 ant upon whose caie and zeal, in that share of the undertaking 

 which fell to his lot, I could safely rely. The unsettled state 

 of the weather, which had indeed been remarkably bad, being 

 no longer an impediment to its commencement, or to its pro- 

 gress, I no longer delayed imparting to my astronomical friends 

 a more accurate account of the new series of observations. The 

 first observations with the new instrument soon showed that 

 the loss of time caused by the reading of the four verniers, and 

 of the level, stands in need of much correction, in order to ob- 

 serve with some degree of accuracy the multitude of stars 

 which crowd through the meridian: even a single vernier and 

 the level cause too much loss on the score of time ; and the 

 neglect of the latter too much on that of certainty. 



I was therefore obliged to think of a method whereby the 

 certainty might be preserved and the consumption of time di- 

 minished. I at length accomplished both by fixing to each 

 pillar of the instrument, a large microscope whose line of vision 

 stands perpendicular to the plane of the circle, and in whose 

 focus the cross wires are moved by a micrometer screw. With 

 these microscopes (on both sides of the instrument) immediate 

 observations may be made of the difference of the zenith or 

 polar distance, and of the zenith or polar distance itself since 

 one of them is known. But as it is not possible to give these 

 microscopes sucli a wide field of vision, as to enable us to per- 

 ceive in all cases on "which line of the circle the cross thread 

 is placed, it was necessary to attach a secondary or assistant 

 arc of 5° of extension (expressly furnished with figures) by 

 means of a clamp to that part of the circle on which the ob- 

 servations are to be made. It was also necessary to contrive 

 an instrument whicli might mark the boundaries of the zojies, 

 and inform the observer, by the stroke of a little hammer, of 

 any excess over these boundaries. M. Fi'auenhofer has had 

 the gootliicsH to construct tliese various additions (which, by 

 means of one reading, make up for (he five which would other- 

 wise be necess;iry) with tlie utmost j)eriection, and in exact 

 conformity witli my plan. 



I'lOlU 



