MICROMETER.] MICRO METER 247 the lens c ; at night the lamp which illuminates the webs and the position circle also illuminates the drum-heads (see on illumination below), aaaois the web-frame (fig. 17), &y is a single rod consisting of two cylinders accurately fitting in the ends of the micrometer box, the larger cylinder being at 0. There is a hole in the web- frame which smoothly fits the larger cylinder at , and another which similarly fits the smaller cylinder at y. A spiral spring, coiled round the cylinder y, resting one end on the shoulder formed by the difference of the diameters of the cylinders and y and the other on the inside of the web-frame, presses the latter continuously towards y. Contact of the web-frame of the micrometer with the side of the box at y would therefore take place, were it not for the micrometer screw. This screw fits neatly in the end of the box at , passes loosely through the web-frame at e, is tapped into the frame at , and its end rests on a flat hardened surface at , Rota tion of the web-frame about fiy is prevented by the heads of the screws at in ; the head of the screw on the lower side of the frame reposes on the plane w, that on the upper side (fig. 17) touches lightly on the inner surface of the lid of the box. Such rotation can obviously be controlled within limits that need not be further considered. But freedom of rotation in the plane of the paper Z I Fig. 17. {fig. 17) is only prevented by good fitting of the holes , y; and, since the weight of the slide is on one side of the screw, misfit here will have the effect of changing the reading for coincidence of the movable with the fixed web in reverse positions of the micrometer. With the Cape micrometer a systematic difference has been found in the coincidence point for head above and head below amounting to 0" 14. This corresponds, in the Cape instrument, with an excess of the diameters of the holes over those of the cylinders of about T^wrth of an inch, a quantity so small as to imply good workmanship, though it involves a systematic error which is very much larger than the probable error of a single determination of the coincidence point. The obvious remedy is to make all measures on opposite sides of the fixed web before reversing in position angle, a precau tion, however, which no careful observer would neglect. In measur ing differences of declination, where the stars are brought up by the diurnal motion, this precaution cannot be adopted, because it is necessary always to bisect the preceding star with the fixed web. But in AS measures index error can always be eliminated by bisecting both stars with the same web (or different webs of known interval fixed on the same frame), and not employing the fixed web at all. Had the spring q been placed as in fig. 14, and the cylinders and y been made to bear like the pivots of a transit on seg- mental bearings in the frame at and y, it is probable that the difference in coincidence points would not have existed. Such a modification appears advisable, unless this construction, by leaving the end m less free, should make the " feel " of the screw less sweet and perfect. The discordance in zero when known to exist is really of no consequence, because the observations can be so arranged as to eliminate it. The box is mounted on a strong hollow steel cylinder CC (fig. 17) by holes i, 6 in the ends of the box, which fit the cylinder closely and smoothly. The cylinder is rigidly fixed in the studs C, C, and these are attached to the foundation plate/. The cylinder contains towards 77 a sliding rod, and towards 9 a compressed spiral spring. There is thus a thrust outwards of the spring upon the hollow cap W (attached outside the box), and a thrust of the rod upon the end of the screw s. The position of the box relative to the plate/, in the direction of measurement, depends therefore on the distance between the end of the screw s and the fixed stud C. A screwing in of s thus causes the box to move to the left, and vice versa. Rotation of the box round CC is prevented by downward pressure of the spring Z on a projection attached to the side of the box. The amount of this pressure is regulated by the screw z . The short screw whose divided milled head is a shifts the zero of the micrometer by pushing, without turning, the short sliding rod whose flat end forms the point cCappui of the micrometer screw at . The pitch of the screw a- is the same as that of the measuring screw (50 threads to the inch), and its motion can be limited by a stop to half a revolution. The five fixed webs are attached to the table rr, which is secured to the bottom of the box by the screws p. The three movable webs are attached to the projections AA on the frame oa. The plane surfaces TT and AA are composed of a bronze of very close texture, which appears capable of receiving a finish having almost the truth and polish of an optical surface. It seems also to take a very clean V cut, as the webs can be laid in their furrows with an astonishing ease and precision. These furrows have apparently been cut in situ with a very accurate engine ; for not the slightest departure from parallelism can be detected in any of the movable webs relative to the fixed webs. Extraordinary care has evidently been bestowed in adjusting the parallelism and distance of the planes r and A, so that the movable wires shall almost, but not quite, touch the sur face T. The varnish to fix the webs is applied, not on the surface T as is usual, but on a bevel for the purpose, 1 the position of the webs depending on their tension to keep them in their furrows. The result is that no trace of "fiddling" exists, and the mov able and fixed webs come sharply together in focus with the highest powers. Under such powers the webs can be brought into apparent contact with such precision and delicacy that the uncertainty of measurement seems to lie as much in the estimation of the frac tion of the division of the head as in the accuracy of the contact. It is a convenient feature inEepsold s micrometer that the webs are very near the inner surface of the top of the box, so that the eye is not brought inconveniently close to the plate when high powers are used. Micrometers of the type E have been invented by Alvan Clark and Clark s Grubb. Clark s micrometer was exhibited at the June meeting of micro- the Royal Astronomical Society in 1859 (Monthly Notices E. A. S., meter for vol. xix.). It is capable of measuring angles up to about one degree, large It is " furnished with two eye-pieces, composed of small single angles, lenses, mounted in separate frames, which slide in a groove and can be separated to the required distance. A frame carrying two parallel spider lines, each mounted separately with its own micro meter screw, slides in a dovetailed groove in front of the eye-pieces ; and by a free motion in this frame each web can be brought opposite its own eye-lens. In using this micrometer, the first step is to set the position-vernier to the approximate position of the objects to be measured. Then the eye-lenses are separated till each is opposite its own object. The frame containing the webs and their micrometer screws is then slid into its place ; and the webs, having been separated nearly to their proper distance by their free motion in the frame, are placed precisely on the objects by their fine screws, the observer s eye being carried rapidly from one eye-lens to the other a few times, till he is satisfied of the bisection of each of the objects by its own web. The frame is then removed for reading off the measure by means of an achromatic microscope, on the stage of which it is placed." The advantages which Clark claims are these : " 1. Distances can be observed with great accuracy up to about one degree, and the angles of position also. "2. The webs, being in the same plane, are perfectly free from parallax, and are both equally distinct, however high the magnify ing power may be. " 3. The webs are also free from distortion and from colour. "4. A different magnifying power maybe used on each of the objects, which may be advantageous in comparing a faint comet with a star." It appears to us that the method of removing a slide in order to measure the interval between the webs is liable to objection, not only because of the risk to the webs, but because the taking of measurements of such a different character with a different instrument is inconvenient and troublesome. It is true that the intervals between the webs could be measured by an assistant, and two or more different slides be employed to save time ; but astrono mers will probably generally prefer the method introduced by Grubb described below. It is understood that Clark has since improved this instrument by an ingenious arrangement of prisms, which permits both webs, even though separated one degree in a large telescope, to be seen in the same eye-piece. The arrangement is not described, and is said to be, as yet, somewhat troublesome to arrange previous to measurement, though when arranged it gives very good results. Grubb (Scientific Proceedings of Royal Dublin Society) thus describes
The marks of varnish so applied will be seen in fig. 17.