Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/260

Rh 244 MICROMETER [TYPES OF FILAR convex sides facing each other. They are placed at a distance apart less than the focal length of a, so that the wires of the micrometer, which must be distinctly seen, are beyond b. 1 The eye-piece slides into the tube cd, which screws into the brass ring ef, through two openings in which the oblong frame, containing the micrometer slides, passes. These slides are shown in fig. 2, and consist of brass forks k and Z, into which the ends of the screws o and p are rigidly fitted. The slides are accurately fitted so as to have no sensible lateral shake, but yet so as to move easily in the direction of the greatest length of the micrometer box. Motion is communi cated to the forks by female screws tapped in the heads m and n Fig. 1. Fig. 2. Fig. 3. acting on the screws o and p respectively. Two pins q, r, with spiral springs coiled round them, pass loosely through holes in the forks k, Z, and keep the bearings of the heads m and n firmly pressed against the ends of the micrometer box. Thus the smallest rotation of either head communicates to the corresponding slide mo tion, which, if the screws are accurate, is proportional to the amount through which the head is turned. Each head is graduated into 100 equal parts on the drums u and v, so that, by estimation, the reading can easily be carried to TTnrirth of a revolution. The total number of revolutions is read off by a scale attached to the side of the box, but not seen in the figure. Two spider webs are stretched across the forks, one (t) being cemented in a fine groove cut in the inner fork k, the other (s) in a similar groove cut in the outer fork Z. These grooves are simultane ously cut in situ by the maker, with the aid of an engine capable of ruling fine straight lines, so that the webs when accurately laid in the grooves are perfectly parallel. A wire st is stretched across the centre of the field, perpendicular to the parallel wires. Each movable web must pass the other without coming in contact with .t or the fixed wire, and without rubbing on any part of the brass- work. Should either fault occur (technically called &quot;fiddling&quot;) it is fatal to accurate measurement. One of the most essential points in a good micrometer is that all the webs shall be so nearly in the same plane as to be well in focus together under the highest powers used, and at the same time absolutely free from &quot;fiddling.&quot; For measuring position angles a brass circle gh (fig. 3), fixed to the tele scope by the screw i, has rack teeth on its circumference that receive the teeth of an endless screw w, which, being fixed by the arms xx to the oblong box inn, gives the latter a motion of rotation round the axis of the telescope; an index upon this box points out on the graduated circle gh the angular rotation of the instrument. The English micrometer still retains the essential features of Troughton s original construc tion above described. The later English artists have somewhat changed the mode of communicating motion to the slides, by attaching the screws permanently to micrometer head and tapping each micrometer screw into its slide. Instead of making the shoulder of the screw a flat bearing surface, they have given the screw a spherical bearing resting in a hollow cone (fig. 4) attached to the end of the box. The French artists still retain Troughton s form. Simms (Troughton s successor) and Cooke (of York), for symmetry and more effectual elimination of &quot;the loss of time&quot; (called by the Germans &quot; todter Gang,&quot; and sometimes in English &quot; back-lash&quot;), have provided two pins with spiral springs, 1 This is known as Ilamsden s rjc-picce; it was made originally by him. like q and r (fig. 2), one on each side of the screw which moves each slide. Grubb of Dublin, with the intention of avoiding the variation of pressure exerted by the spiral springs when the slide is at different distances from the head of the screw, has adopted the following plan. Where the screw enters the slide he has a nut n attached to a strong spring pp (fig. 5), the pres sure of which exerts a con stant tension in the axis of the screw, tending to bring the threads into close con tact, in opposite directions, with their bearings in the nut n and the slide q. The pressure of this spring is regulated by the screws s, s, tapped into the thickened ends of the springs. For maintaining the spherical shoulder of the screw in close and constant pressure on its conical bearing he has attached a conical bear ing to the spring p p (fig. 6). The pressure of this on the upper part of the spherical shoulder is regu lated by the screws s, s , passing through elongated holes in the spring p p, and tapped into the end of the box. The screws of micrometers are generally made with 50 or 100 threads to the inch. Troughton s method of reading the number of whole revolutions by a silver scale is inconvenient, because r ^th or even -^th of an inch is too small a quantity to read easily with the naked eye, especially with the faint illumination that it is desirable to use when measuring faint objects. Different methods, including the &quot;comb &quot; (see below) and various kinds of &quot; counters,&quot; have been introduced with more or less success ; but recently the Repsolds of Hamburg have contrived a plan at once s&amp;lt;i simple and so efficient that Fig. 7. it will be unnecessary to describe those methods which this plan is certain to super sede (see below, type D). Grubb has introduced a modification in the form of & the slides with a view to avoid the friction of one slide against the other. On the inner side of the brass plate which forms the bottom of the box (i.e., the side opposite to the eye-piece) four V-shaped furrows are placed (fig. 7); and at each end of the slides are pro jections (fig. 8, end view) which fit into these furrows. The slides are kept down in their places by springs attached to them, which press upon the inner side of the lid of the box. Troughton s mode of giving rotation to the position circle is now abandoned. A much quicker motion in position angle than can be obtained without slow motion is often desirable, since, in observing very close double stars, the uncertainty of each point ing may amount to several degrees in the most accurate measure ments. The plan of a pinion working in a toothed wheel is often employed, but that also is too slow. Most modern micro meters are now fitted with a clamp and slow motion screw (see fig. 9, type B). This permits observation of position angles of veiy close objects by simple rotation of the box with the hand ; while the slow motion, after clamping, permits the more delicate movements that arc required in measuring the position angle of objects farther apart. The Cookes and Grubb have for years almost invariably trans ferred the position circle from the micrometer to the telescope tube. The whole eye-end with its focussing arrangements rotates, and its rotation can be measured by a circle attached to the butt end of the tube. There is considerable convenience in this arrangement. One position circle only is required for all the micrometers that may be employed with the instrument ; and the orientation of reticulated diaphragms, or the adjustment of the direction of the slit of a spectroscope, may also be accomplished by the same means. But, after a very extended experience of all the various types of existing mountings, the present writer does not hesitate to express a decided preference for a position circle attached to the micrometer and a rigid attachment of the eye-end to the telescope tube, having never seen an eye-end attached to a position circle on the butt end of the telescope-tube in which, after the wear and tear of a few years,