A Substitute for the Position Micrometer. 44-9 



therefore it is graduated by long lines to every 10°, and snorter 

 lines for every 5°, and by dots to 2|° (see Fig. 1). 



This all-important part of the contrivance being duly 

 attended, the index by which it is to be read next comes iu 

 for consideration. This is of steel. Its form will be more 

 readily comprehended from the drawing than from any written 

 description. It is attached to the eye-tube by the brass screw 

 and nut, which is pierced with a slit lengthwise for its recep- 

 tion and adjustment, the length of the index being so managed 

 that when the instrument is adjusted to focus, its point just 

 reaches the circle. The screw on lower side of eye-tube is 

 merely to assist in revolving the eye-piece. 



Inside the eye-piece, in the focus of the eye-lens, I have 

 fixed a perforated cardboard disc, the opening being rather 

 larger than the aperture of the stop always placed there, and 

 across the field a conspicuous wire is carefully placed (see 



Now to render this visible during observations it needs 

 some illuminating apparatus, and I have used with advan- 

 tage an oval reflector of bright tin, with a sufficient 

 central aperture to allow all the rays from the object-glass to 

 reach the eye unobstructed. This is fixed inside the tube, at 

 an angle of about 45°, as shown in Fig. o. The relative posi- 

 tion of the screw, and the hole for admitting light, are shown 

 in Fig. 4. This hole needs to be no larger than a sixpence, 

 and should be just opposite the point a, Fig. 3. For a lantern 

 I nse a tin canister, about seven inches high, and two in 

 diameter, with an opening in its side for light, and numerous 

 perforations in the top for the escape of heat. Within this is 

 a small paraffin lamp (see Fig. 5). The degree of light, and 

 its distance and position with respect to the aperture in the 

 telescope, will be better learnt by a few practical lessons than 

 . by any written instructions. 



Having detailed the construction of the apparatus, we will 

 now point out its use, and for this purpose we cannot do 

 better than suppose a pair of stars in the field, as shown in 

 Fig. 6. Now to measure their angle of position, revolve the 

 eye-piece till the index is to the left side of the circle, and 

 bringing the larger star to the edge of the wire, gradually 

 revolve the eye-tube till the star in its motion through the 

 field exactly traverses the wire. Note the degree indicated 

 on the circle. This will be 270°. Now revolve the tube till 

 the wire becomes parallel to the pair, and note*the number of 

 degrees it has turned, as from 270° to 220°. This is 50°; but 

 as the smaller star is on the opposite side, the reading must 

 likewise be transferred, and you must substract 50° from 90°, 

 which leaves 40° for the angle. 



VOL. VIII. — NO. VI. GG 



