Topographic surveying; including geographic, exploratory, and military mapping, with hints on camping, emergency surgery, and photography . Fig. 162,—Eight-inch Direction Theodolite. 558 FIELD-WORK OF PRIMARY TRIANG U LATION. or films. These are similar to the cross-hairs of the eye-piece of a telescope. The instrument consists of tliree sepa-rate parts: 1. The microscope tube, carrying the lenses for magnify-ing the divisions on the circle and the hairs; 2. A large-headed screw the outer circumference of whichis divided, and is read by means of a fixed pointer; and 3. A comb-scale and cross-h
Topographic surveying; including geographic, exploratory, and military mapping, with hints on camping, emergency surgery, and photography . Fig. 162,—Eight-inch Direction Theodolite. 558 FIELD-WORK OF PRIMARY TRIANG U LATION. or films. These are similar to the cross-hairs of the eye-piece of a telescope. The instrument consists of tliree sepa-rate parts: 1. The microscope tube, carrying the lenses for magnify-ing the divisions on the circle and the hairs; 2. A large-headed screw the outer circumference of whichis divided, and is read by means of a fixed pointer; and 3. A comb-scale and cross-hairs by which the divisionsof the circle are read and subdivided. The micrometer cross-hairs arid comb-scale are fixed in theplane of the image produced by the objective of the micro-. FiG. 163.—Section of Micrometer through Screw showing CombAND Cross-hairs in Central Plan. scope. This image is larger than the object seen in themicroscope, therefore a given amount of the micrometercross-hairs corresponds to a much less distance on the ob-ject sighted. The cross-hairs are held in a frame which ismoved by a screw having a very fine thread, called the mi-crometer screw. (Fig. 163.) This is caused to revolve bya large head, called the tnicrometer head, which is cylin-drical or hollow, its outer circumference being divided intosixty. The relation between the comb-scale of the microscope,and the graduations on the micrometer head which denotethe fractions of a revolution of the screw, is such that onefull revolution of the screw corresponds to one tooth of the MICROMETEK MICROSCOPE. 559 comb-scale. The number of whole revolutions of the screware recorded by noting how many teeth of the comb-scaleare passed over; the fractional parts of a revolution beingread on the gra
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