OPPOSITION OF MARS: 



1849-50, 



OBSERVATIONS MADE AT THE ROYAL OBSERVATORY, CAPE OF GOOD HOPE. 



8i-FEET EQUATOKIAL MICROMETER. 



This micrometer is provided with two parallel spider lines, 0".94 of a great circle in thick- 

 ness. One of these is called the zero wire, and is moved by a screw without a register-head. 

 The other, or micrometer wire, is moved by a screw whose head is divided into 100 parts, and 

 the range is 60 revolutions. The revolution scale is placed conveniently outside; and by the 

 motion of the micrometer screw the eye-piece is carried along, so that the wire is kept in the 

 centre of the eye-piece field. The rack-edge which moves the eye-piece slide is limited to the 

 range of 30 revolutions of the micrometer screw; so that, when the zero wire is near one edge 

 of the field, (with the lowest power 123,) the micrometer wire is at the other edge. 



When making observations for discovering the run of the screw, the wires were placed 20 

 revolutions apart. 

 By 40 transits of equatorial stars observed near the meridian over the centre, 20 



revolutions, corrected for refraction in right ascension, 1 revolution . . . . = 26". 0699 



By 30 transits over the same interval = 26". 0698 



By 20 transits, from revolution 10 to revolution 30, and from 30 to 50 . . . = 26 V .06715 



The mean is 26". 06925 or 26' 4".155 for the 60 revolutions. Mean of the temperatures 

 during observation about 66. 



These observations were made on three different nights. 



The readings of the screw-head and revolution scale increase on turning the screw backwards, 

 which order carries the wire in the direction of the screw-head. 



EQUATORIAL ADJUSTMENTS. 



Before the commencement of the observations on Mars, a point in the wire was stationary 

 during a revolution of the position verniers, when the reading of the screw-head was 30.14 rev. 



The collimation error in right ascension was 0.3s. 



The inclination of the declination to the polar axis, 16". 4. 



The excentric co-ordinates of the south polar pivot, x= 10". 1 towards the zenith, y= 0".9 

 towards the west. 



Declination circle reading for the polar point, 270 0' 23". 5. 



At the termination of the observations, the position circle verniers rotated round a point in 

 the wire, when the screw-head reading was 30.15 rev. 



The collimation error in right ascension, 1.07s. 



The inclination of the declination to the polar axis, 8". 8. 



