A treatise on surveying, comprising the theory and the practice (1906)
Gillespie, William Mitchell, 1816-1868, Staley, Cady, 1840- · New York D. Appleton · leaf 53
e usually cut on a thin piece of glass and blackenedby rubbing a little powdered plumbago across them. This thin 42 6E0DEST. piece of glass is then known as a diapliragm. The diaphragm isused as little as possible, since it always makes the image appearfainter, no matter how thin or clear it may… … I may be used in low-powered telescopes where the pointings arenot very exact. II is preferred on theodolites used on tertiary trian-gulation, or occasionally on the secondary triangulation, particularly inthe repeating instrument. Ill is used on telescopes or microscopes,where lines are to be…
Surveying
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