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Figs. 128, 129.—Sections of scales and vernier for circular readings.
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324.—Surfaces of Limb and Vernier.—To get a perfect reading of a vernier the scale and vernier should be brought into contact upon a plane. This, for many reasons, is impossible in a great number of cases upon an instrument, from the conditions of its construction, convenience of vision, and in some cases for want of means of ensuring durability of the edges which work together. Therefore verniers and scales are more commonly constructed upon the methods shown in section Figs. 126, 127, where VV are verniers, LL limbs. The plan shown in section Fig. 128 gives a nice reading on a new instrument; but the part of the edge not covered by the vernier is open to accident, or if nearly covered by a part of the instrument, open to the introduction of gritty dust, which wears the meeting line open, and thereby causes loss of edge to edge reading. Fig. 129 shows a section we find on some French instruments. This plan was introduced by the late Colonel A. Strange for the section of the limb reading of theodolites for India, but it was found in practice awkward to use upon this instrument, as it required unpleasant stooping to read it. It is, nevertheless, one of the best permanent vernier readings, as the division remains constant under the amount of wear occasioned by the sliding of the vernier upon its circle.
325.—With the reading planes shown in section Fig. 126 we require great care to bring the eye, whether open or through the microscope, directly radial with the centre of the circle at the line into which the vernier cuts. If we read the line in the slightest degree one-sided it is quite possible to make a difference of a minute on a 5-inch or 6-inch circle. This is the section of the general reading plane of theodolites, where, from the necessary height of the telescope, the limb has to be placed much lower than the eye. With this section the circle comes fairly square to a comfortable position for reading. It will be noticed that there is a slight lap shown to the vernier over the limb at a, Fig. 126, which is always found in new instruments of this section. It gives an allowance for wear between the vernier and the limb caused by the fretting of the metals together, as also by the intrusion of grit, which is always present in instruments used in the open air. The lap should not be great, and it should be nearly equal along the edge of the vernier, although it is a difficult matter for the maker to get it perfectly so.
Fig. 127 is a section of the reading planes common to sextants and parts of many instruments. This plan requires the same care to obtain a truly perpendicular reading to the division as that described above for Fig. 126.
326.—In the very best of work there is at all times a certain amount of error, both between the divisions themselves, and in the place of the axis in relation to the centre of the divided circle, and of the position of the vernier in relation to both these. It therefore becomes necessary, where exactness is required, to place at least two verniers to read opposite sides of the circle. These bisect every reading through the axis of the instrument, and detect very small errors in the work, as well as personal errors of the observer, of which the mean reading of the minutes or seconds only may be taken and used for correction to mean position. Where very great precision is aimed at, three or even five verniers are sometimes placed round the circle, and the mean reading is taken of the small differences in minutes or seconds, after calculation for correction, to find the direct position of the axis of the telescope required for the record of the observation.
327.—Reading Microscope.—The microscope usual for reading the vernier is either a simple plano-convex lens of short focus or a Ramsden eye-piece of the kind described for observing lines on the diaphragm of a telescope, [art. 82]. Frequently the microscope, technically called the reader, is made of a compound form, sometimes with a diagonal prism or mirror. It is uniformly mounted in such a manner that it may move concentrically to the divided circle into which it reads. In English instruments it is placed normal to the surface of the vernier, so that following its curvature it may read opposite any line upon it. In French instruments the reader is frequently placed obliquely, so as to look along the line of the limb into that of the vernier, which is said to be advantageous in certain lights.
328.—In theodolites for reading the horizontal circle, the reader is sometimes mounted to slide in an undercut groove near the circumference of the limb to follow its curvature. This motion is not pleasant; it is better in this and all cases of vernier reading, if possible, to mount the reader on frame-work proceeding directly from and moving upon the axis. Where it is practicable, it is much better to have two readers where there are two verniers, and in all cases to have one to each vernier, than to shift one reader about after the instrument is placed in position, which is liable to disturb it. With opposite readers mounted on a pair of arms formed of one piece of metal, where these bisect the circle working through its axis, by the setting of one reader truly normal to the coincident division of the vernier the opposite reader will be set also; so that this does not only save time, but the instrument need not be touched for reading the second vernier. The same principle should be applied to any greater number than two verniers as nearly as it may be practical.