464 OPPOSITION OF MARS, 1849-50, 



" Like the position-micrometer on the English construction, this has two parallel movable 

 wires ; but one of the screws only is provided with a register-head. The other is simply a zero wire, 

 and is subservient to either the cross measure or to measurement by repetition. Both construc- 

 tions have a dovetail slide for the eye-lenses, worked by a rack and pinion ; but independent of 

 this the Munich register-slide carries the eye-lenses with it, by which the wire is kept in the 

 centre of their field ; this is a decided advantage. 



" When the telescope is pointed towards the zenith, it is much more convenient for the ob- 

 server to have the divisions of the position-circle on the edge (mural-circle fashion) than on the 

 face, as in the Munich construction. 



" The declination-circle is about 12J inches in diameter, and is read by means of two opposite 

 verniers to 10"; but less than half this quantity can be estimated. The degrees are numbered 

 from to 360, and when the telescope is west of the pier, and pointed to the equator, the 

 vernier reads 0. 



"The right ascension, or hour-circle, is about 9.6 inches in diameter. The hours are num- 

 bered from Oh. to 24A., and the subdivisions are read by means of two opposite verniers to 4s. ; 

 but single seconds can be estimated with facility. Oh. is parallel to the telescope end of the 

 declination axis ; and telescope east of the pier, the readings increase with the diurnal motion 

 of the heavens. 



" The driving clock performs its duty remarkably well, and without noise or tremor. The 

 head must be placed in contact with the box to hear it. Its construction is similar to that 

 attached to the great refractor at Pulkova. 



"Few direct experiments have been made to determine the quality of the object-glass ; but 

 these have proved satisfactory. The two last stars in the trapezium of Orion are instantly 

 visible, even in bright moonlight. 



" The companion to * Orionis is seen under favorable circumstances. Bright stars are round 

 during good atmospheric conditions, and under high powers are surrounded by well-defined 

 rings. The companion to Antares is well separated from its primary under favorable circum- 

 stances, and the measures appear to be little inferior in precision to those of Centauri. There 

 is a slight tint of blue towards the margin of the field when a bright star is in the centre. The 

 binary y Coronas, Australia presents two bright dots, separated by a black line. Their distance 

 is less than 2". 



"The telescope tube was proved to be stiff when it became necessary to employ this instru- 

 ment for obtaining an approximate determination of the declinations of faint stars compared with 

 Metis, the mural circle being (optically) unequal to the task with the ordinary method of 

 'down-tube' illumination. To test the flexure of the equatorial telescope tube, twenty stars 

 were selected from the Nautical Almanac between right ascension 0/t. and 7/t., and in declina- 

 tion from Persei to Trianguli Australis S. P. The observations were made near the meridian, 

 and each star was observed in reversed positions of the telescope. At the time, the adjustments 

 were as follows : The micrometer rotated on a point in the wire when the screw-head registered 

 30.14 rev. The collimation error in right ascension was 3" ; the inclination error of the decli- 

 nation to the polar axin 1C". 4. The co-ordinate deviations of the instrumental south polar 

 pivot from the celestial pole were x = 10". 1 towards the zenith, y = 0".8 towards the west. 



"Assuming the tabular declinations to be correct, the result for the constant of flexure was 

 0".4. That the quantity was so small is, no doubt, partly accidental, because the probable 

 error was greater, and because the circle readings canno.t well be estimated below 3", although 

 the divisions are remarkably clear and distinct. 



"The application of an instrument of this construction to the purpose above mentioned, is 

 justified by necessity alone. But it is proper to explain that zero stars were substituted for 

 li-vi-ls, &c. Then, making due allowance for the disproportion between the optical power of the 

 t-l-' ,,|,i- and the dimensions of the declination-circle, if the relation between the optical axis of 

 one and the verniers of the other remain constant for a few hours, with a firm pier and stiff 



