A treatise on surveying, comprising the theory and the practice ; (1887)
Gillespie, W. M. (William Mitchell), 1816-1868, Staley, Cady, 1840- [from old catalog] ed · New York, D. Appleton and company · leaf 366
reads.They are attached to a short, thick tube,placed within the telescope-tube, through whichpass loosely four screws whose threads enterand take hold of the cross-hair ring, as shownin Fig. 366. In some instruments, one of each pair ofopposite screws is replaced by a spring ; and the screws,… … (or line of aim) is theimaginary line passingthrough the intersection ofthe cross-hairs and the op-tical center of the object-glass. The image formed bythe object-glass should co-incide precisely with thecross-hairs. When this isnot the case, there will be an apparent movement of the cross-hairs,…
Surveying
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