TRANSIT OBSERVATIONS. THE ALTAZIMUTH. 489 



THE ALTAZIMUTH INSTRUMENT AND OBSERVATIONS. 



The Altazimuth used at Burnhain was made by Messrs. Troughton and 

 Simms about the year 1864 for the Royal Observatory. It has two vertical 

 and one horizontal circles of 15 inches diameter, divided to 5', each read by 

 four micrometer-microscopes. The object-glass is of 2| inches diameter and 

 27 inches focal length. The telescope and horizontal axis are constructed to 

 give what is known as the axis-view. A rectangular prism of total reflexion 

 is placed at the point of intersection of the optic axis of the object-glass with 

 the horizontal axis, so as to reflect the cone of light formed by the object- 

 glass horizontally into one of the pivots to which the eye-piece and reticule 

 are fitted. The weight of the half-telescope carrying the object-gass is 

 counterpoised. The illumination of the field of view is effected by lamp-light 

 passing through the opposite pivot and through the center of the prism 

 mentioned above, a very small rectangular prism being cemented to the center 

 of the inclined face of the larger prism, and, therefore, exactly in the line of 

 pivots, to allow the light to pass through. There are five vertical and five 

 horizontal wires, the latter having larger intervals than the former. 



The microscopes of the horizontal circle are supported by radial arms cast 

 in one with the rotating body of the instrument, and are perfectly rigid. 

 There are two zenith-distance levels and two levels attached to the horizontal 

 circle. A striding level, similar to those used with Transit instruments, is 

 used for determining the inclination of the axis. The value of the divisions 

 of this level were tested before leaving England, when 50 divisions were 

 found equal to one minute of arc, which was the former value. 



On unpacking the instrument at Burnham, one of the pivots, which were 

 of hard white steel, was found to be badly rusted, apparently from a piece of 

 paper having been wrapped round it; for the other pivot, without paper, was 

 quite bright. A small spider had been at work among the webs of , the 

 reticule, but without doing any real injury to them. The pivot having been 

 cleaned as well as possible, the instrument was mounted, on October 16, on a 

 pier of masonry. 



The inequality of the pivots of the horizontal axis was approximately 

 determined on December 11. The level error was determined five times with 

 the axis in each position ; the resulting correction to the level error found 

 by the striding level, 0"'62, positive with " lamp right," has been applied 

 throughout. 



SR 2 



