Inslnimental Error of his Horizon-Seclor. 339 



mahogany frame (D) about two feet long and 2*5 inches 

 square, each end of the frame projecting equally beyond its 

 subjacent block. The sidereal focus of the object-glass was 

 full 20 inches, forming with the eye-tube a total focal length 

 of nearly 30 inches. Within the eye-tube, between the first 

 and second glasses, was placed a stop containing a vertical 

 slip of very thin mother-of-pearl divided on one edge by 

 fine horizontal lines into equal parts of about 1' 20" each, 

 together with two fine spider threads crossing each other, a 

 few seconds to the east, and in the plane of the slip of pearl, at 

 an inclination to the horizon of about 30°. By means of 

 the micrometer screw 

 (H), worked by the 

 milled head-nut (F) 

 placed outside the 

 tube, the intersecting 

 point of the threads 

 could be gradually 

 moved upwards or 

 downwards, parallel 

 to the divided edge 

 of the Jixed slip of 

 pearl. The handle 

 (G) served to slide 

 the whole apparatus, 

 parallel to the sides 

 of the eye-tube, to 

 that distance from 

 the eye-glass at which 

 the threads appeared 

 most distinct. Four screws, with sliding slips of brass be- 

 neath their heads (of which two (I 1) are represented in the 

 diagram), were used in the usual manner to alter the situation 

 of the point of intersection of the threads. 



The proper object-glass (I) of the sector, fitting tightly 

 within the thicker end of the cylinder, is 13*1 inches in focal 

 length, and protrudes about 2 inches beyond the cylinder. 

 Within the opposite end of the cylinder, the eye-piece being 

 withdrawn, was fixed an additional object-glass (J) of 10 '3 

 inches focal length, protruding about 4 inches. The stop 

 containing the (adjustable) horizontal and vertical wires is 

 fixed within the cylinder, and was made the joint or common 

 focus of the two object-glasses. The sector being placed about 

 the middle of the length of the plank, with its cylinder nearly 

 on a level, and in a line with the telescope (1)), the stand of 

 the sector was moved laterally until the vertical wire of the 



2 X 2 cylinilcr, 



