A treatise on surveying, comprising the theory and the practice (1906)
Gillespie, William Mitchell, 1816-1868, Staley, Cady, 1840- · New York D. Appleton · leaf 41
ays are reflecteddirectly along the central line or not, the ring next to the mirrorhas an opening slightly larger than that in the outer ring; hence,when the beam of light is central a narrow strip of light will bevisible all around the opening of the outer ring, and all the hclio-troper needs to… … HELIOTROPES. 31 that the central line of the rings is parallel to the line of collimationof the telescope. After making the central line of the rings andmirror as nearly parallel with the telescope as possible by sightingalong the side, direct it toward some distant and well-defined object(either…
Surveying
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