142 Proceedings, <&c., for 1883. 



measuring web is actuated by a screw with 100 threads to 

 the inch, and is ' kept to its work' by a pair of fine spiral 

 springs, one on each side of the screw. The smaller frame 

 carries eight webs, three parallel to the screws and five at 

 right angles to it. This frame is used for adjusting to zero 

 only, and is therefore movable over a very small range by 

 means of a short screw of 100 threads to the inch, also 

 opposed by a strong spiral spring. Three bisecting wires 

 are used to avoid too great a range of the measuring screw. 

 The whole micrometer box is movable at right angles to the 

 optical axis of the telescope by means of a well-made slide, 

 actuated by a screw with 60 threads to the inch, opposed by 

 a strong spiral spring. The eye-piece is made to slide 

 across the field of view on the micrometer box by means of 

 a quick four-threaded screw. The measuring screw has an 

 epicyclical count wheel-head, as well as the divided head 

 proper, so that whole and parts of revolutions of the screw 

 are read at once. The slide for the eye-piece, as well as 

 that for the whole micrometer box, is moved by two 

 milled screw-heads (the eye-piece one being much the 

 smallest), situated at the end of the box opposite the micro- 

 meter screw-heads, and is very convenient for manipula- 

 tion. 



" This instrument can be used as an ordinary micrometer 

 with a bright field by using the light from the central 

 reflector in the telescope. It is in the arrangement for dark- 

 field illumination that the principal novelty exists. The 

 micrometer box, with sliding stage, is attached to a piece of 

 tube 4 inches long, terminating in the adapting screw for 

 attaching to the telescope. At right angles to this tube, 

 and about 2J inches from the micrometer box, is fixed 

 another short tube, somewhat smaller, to carry one of Swan's 

 2|-candle-power incandescent lamps. The light from this 

 lamp, rendered parallel by means of a lens, falls on diagonal 

 mirrors of silvered glass, and is thence reflected toward the 

 micrometer box, filling an annular space between the outer 

 tube and a smaller short one to prevent stray light entering 

 the field of view. The light then passes through four square 

 apertures at the base of the micrometer slide, two being 

 parallel with the slides, and two at right angles to them. 

 The holes communicate with small rectangular boxes, which 

 conduct the light to four small thin glass mirrors adjusted 

 to reflect the light exactly in the plane of the wires, 

 illuminating both systems symmetrically on both sides. 



