Topographic surveying; including geographic, exploratory, and military mapping, with hints on camping, emergency surgery, and photography . instruments have a diameter of 8 inches and are dividedto 10 minutes, though they can be read to 2 seconds of arcby means of two micrometer microscopes placed on oppositesides of the alidade. The object-glass is 2 inches in diameterand has a focal length of 16^ inches, with an eyepiece havinga magnifying power of about 30 diameters. 242. Micrometer Microscope.—This is a device for themeasurement of smaller parts of an arc than are indicated bythe graduatio
Topographic surveying; including geographic, exploratory, and military mapping, with hints on camping, emergency surgery, and photography . instruments have a diameter of 8 inches and are dividedto 10 minutes, though they can be read to 2 seconds of arcby means of two micrometer microscopes placed on oppositesides of the alidade. The object-glass is 2 inches in diameterand has a focal length of 16^ inches, with an eyepiece havinga magnifying power of about 30 diameters. 242. Micrometer Microscope.—This is a device for themeasurement of smaller parts of an arc than are indicated bythe graduations upon it. Micrometer microscopes are usedin high-power angle-reading instruments in place of the ver-niers used on engineering instruments, since they give moreaccurate results and finer subdivisions of the arc. They con-sist of a microscope generally supported upon the standardsof a theodolite with the objective end in close contact withthe horizontal circle, and lighted by a cylindrical glass ex-tension. It is sometimes called the Jilar inicrometer, becausethe small measurements are made by means of fine threads THEODOLITE, 557. 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-
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