For the smaller instruments which will be much more frequently used by the surveyor a simpler form of reading is used, and as the reviser is convinced that in future this form of reading will gradually replace the vernier for all high-class work, a full description of this very simple reading is here given. The reviser is confident, after many years of practice for the most accurate form of index, that a point certainly stands first, a pair of webs or lines on glass, between which the division is seen, second; and a single web or line on glass placed over the division, third. The comb mentioned in art. 339 is done away with, and one revolution of the micrometer screw made to carry the index over one division of the limb. For clearness the engravings show only a 10″ reading; for a 5″ reading the divisions on the limb are to 5′ instead of 10′, and the micrometer head is divided and figured accordingly.
Fig. 140.—Stanley's micrometer reading.
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Fig. 140 shows at C a portion of the theodolite circle as seen through the micrometer microscope. P is the movable pointer, M the micrometer head, and I the index or reading line.
To use the micrometer the first steps are to carefully focus the pointer P by means of the eye-piece until it appears clear and perfectly sharp, and set the reflector at the bottom of the microscope so that it reflects sufficient light to illuminate the divisions on the circle. Then, by turning the micrometer head M, set the pointer P to the centre of its travel, so that it covers the V cut in the bottom of the slide, and leave the 0 of the micrometer head exactly opposite the index line I. Now proceed in the same manner with the other microscope. After setting the microscopes as described above, lightly clamp the lower clamp screw of the instrument and release the upper one. Now revolve the upper part of the theodolite until 360 degrees on the circle appears exactly under the pointer of one of the microscopes. The other will then be pointing to 180 degrees, and the instrument is set ready for measuring the first angle.
Fig. 141.
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We will presume now that a bearing has been taken by the telescope and it is required to read the angle, and that on inspection of the micrometer it is seen to be in the position illustrated at Fig. 141, viz., between 227 and 228 degrees. Now as the degree is subdivided into 6 parts, each of these subdivisions must represent 10 minutes of arc, therefore the pointer is situate between 227° 30′ and 227° 40′. It is now necessary to measure exactly the distance of the pointer from the division 227° 30′, which is done in the following manner, by means of the micrometer head M.