MICROMETER.] M ICKOMETE R 245 some looseness or shake couM not be detected. This is a fatal fault, especially in those delicate observations of difference of declina tion which have latterly formed so prominent a feature in refined micrometric research. On the other hand, in some good old micro meters at the Royal Observatory, Cape of Good Hope, that are fitted with attached position circles, there is no trace of shake or wear after fifty years of work. The micrometer of type B represented in fig. 9 l is the original Merz micrometer of the Cape Observatory, made on Fraunhofer s model. S is the head of the micrometer screw proper, s that of the screw moving the slide to which the so-called "fixed web" is attached, s that of a screw which moves the eye-piece E. C is the clamp and M the slow motion in position angle. L, L are tubes attached to a larger tube N ; the latter fits loosely on a strong hollow cylinder which terminates in the screw V. By this screw the whole apparatus is attached to the telescope. The Dunsink micro meter. Fig. 9. nozzles of small lamps are inserted in the tubes L, L, for illu minating the webs in a dark field ; the light from these lamps is admitted through apertures in the strong hollow cylinder above mentioned (for illumination, see below). In this micrometer the three slides moved by S, s, and s are simple dovetails. The lowest of these slides reposes upon a foundation-plate pp, into one end of which the screw s is tapped. In the middle of this slide a stiffly fitting brass disk is inserted, to which a small turn-table motion may be communicated by an attached arm, acted on by two fine opposing screws accessible to the astronomer ; and by their means the "fixed wire" may be rendered strictly parallel with the movable wire. The micrometer screw is mounted on the slide which carries the movable web. Fig. 10 shows a plan of this slide; the divided drum of the screw is omitted for sake of clearness. The screw S has a shoulder at K, carefully fitted and ground to a bearing so as to work sweetly in a hole in the very strong spring ffa", the other extremity of the screw is formed into a pivot, which fits a hole in the brass piece /3/3. The end of this pivot hardened, polished, and slightly rounded rests on the flat surface of an agate a, which is imbedded in the end of the slide, and kept firmly in its place by the brass piece . By careful adjustment of the screws 6, sufficient pressure may be left upon K to slightly bend the strong spring ffcr and thus eliminate all end-shake without preventing easy action of the screw. The screw passes at the same time through the bush B (shown in plan and elevation, fig. 10) attached to pp (fig. 9); and there is a fine saw cut, which can be narrowed by the small screw r, to close the bush upon the micrometer screw with a view of preventing "loss of time." The spider web co is cemented on the further side of the thin plate vw, the varnish being applied in the countersunk holes shown by the dotted circles /JL, /j.. The slide is counter sunk to about half its thickness within the area indicated by oooo, in order to allow the adapter of the eye-piece to come sufficiently close to the webs. The eye-piece was origin ally moved by a pinion working in a rack ? (fig. 9); but the screw s applied by Simms was found by Maclear to be more convenient for the purpose. Beyond this, and the grad uation of the edge of the circle with more strongly cut divisions than those originally engraved on the face of the circle, the instru ment remains and is figured in its original form. Pistor and Martins (Berlin) have also made excellent instruments of the above type. There is a celebrated micrometer of their make, with which, in the hands of Brunnow at Dunsink (Dublin), some of the most perfect and refined investigations ever made in practical 1 When it is remembered that the measurements of the Struves, Dembowski. Secchi, the Bonds. Mncluitr, and of most modern Continental astronomers have been made with Fraunhofer or Merz micrometers, it is not too much to say that tK- 9 represents the instrument with which three-fourths of the astronomical measurements of the >ast fifty years have.been made. astronomy have been executed. In this micrometer the screw s is mounted on its own slide and has a divided head precisely like the screw S (lig. 9). The plate pp is elongated towards s, and the corresponding bush B is attached to this elongation. The screw sf is shifted to another part of the eye-piece slide, so that it does not interfere with the increased diameter of the screw s. Fraunhofer s micrometer in this form belongs to type A, but is quoted under type B for convenience of description. It is not necessary to give a figure representing type C. Such micrometers have been generally constructed on Troughton s type (figs. 1, 2, 3) with the omission of one of the screws, and with one or more of the modifications described in detail under type A. Some have also been made similar otherwise to the Fraunhofer con struction, by omitting the screw s with its corresponding slide and attaching the fixed wire to a circular plate in pp. Good instruments have been made on type C by Clark (Cambridge, Clark s Massachusetts), by Steinheil (Munich), and by the great French filar artists Secretan, Froment, Brunner, Eichens; andgoodwork has been micro- done with them. But it is necessary that the errors of the screw meter, should be very carefully determined, since, in type C, such errors cannot be eliminated by employing different parts of the screw to measure the same angle. There is a noteworthy description of micrometer that forms a link between types C and D, of which the most famous example (by Clark) is attached to the great Washing ton telescope. It is essentially a micrometer of type C, with a slide (or fork) and a screw of the English form of construction. But the instrument is provided with a screw as at s (fig. 9), which, instead of changing the position of the fixed wire, moves the whole micro meter box in the direction of the axis of the measuring screw. Thus the fixed wire can be set exactly on one star by the screw s while the other star is immediately afterwards bisected by the movable wire, and that without disturbing the reading for coincidence of the wires. No one, unless he has previously worked without such an arrangement, can fully appreciate the advantage of bring ing up a star to bisection by the fixed wire by moving the micrometer box with a delicate screw-motion, instead of having to change the direction of the axis of a huge telescope for the same purpose. When it is further remembered that the earlier telescopes were not provided with the modern slow motions in right ascension, and that the Struves, in their gigantic labours among the double stars, used to complete their bisections on the fixed wire by a pressure of the finger on the side of the tube, one is puzzled whether most to wonder at the poor adaptation of means to ends or the marvellous patience and skill which, with such means, led to such results. 2 It should be added that Dawes practically adopted a modification of Clark s micrometer by using a slipping piece, and bolting one of the heads of his micrometer (Mem. 11. A. S., vol. xxxv. p. 139). His slipping piece gave motion to the micrometer by two slides, one in right ascension the other in declination, so that " either of the webs can be placed upon either of the components of a double star with ease and certainty." All micrometers used, in conjunction with a microscope, for read ing the divisions of transit circles, heliometer scales, &c., are of the type C. The reading micrometer- is shown in fig. 11. C is the objective, D the micrometer box, E the grad uated head of the screw, G the milled head by which the screw cc is turned, A an eye piece sliding in a tube B, aa (fig. 12) the slide, F J Fig. 11. Fig. 1-2. and b, b the spiral springs. The focal length of the objective and the distance between the optical centre of the lens and the webs are so arranged that images of the divisions are formed in the plane of the webs, and the pitch of the screw is such that one division of the scale corresponds with some whole number of re volutions of the screw. There is what is technically called a " comb " inserted in the micrometer box at d (fig. 12), its upper surface being nearly in the plane of the wires. This comb does not move with reference to the box, and serves to indicate the whole revolution of which a fraction is read on the head. In fig. 12 a division is represented bisected by cross webs, and five revolutions of the screw correspond with one division of the scale. In all modern reading micrometers the cross webs of fig. 12 are replaced by parallel webs embracing the division 2 The late Piofessor Watson used to say. quaintly and with truth, "After all,
the best part of th micrometer is the man at the small tnd! "