The American annual of photography . en readoff on the graduated scale. The telescope is now transferredto the other end of the baseline, the angle which it makes whenthe staff is sighted being again recorded. The surveyor hasthen obtained three factors which, taken together, infalliblygive him the distance and exact position of the staff, namely,a measured baseline with an angle at each end. He has onlyto draw the baseline on the plan, to any convenient scale, andto make similar angles at the two ends. These angles, whenproduced, will meet each other, forming a triangle, which willshow to sca
The American annual of photography . en readoff on the graduated scale. The telescope is now transferredto the other end of the baseline, the angle which it makes whenthe staff is sighted being again recorded. The surveyor hasthen obtained three factors which, taken together, infalliblygive him the distance and exact position of the staff, namely,a measured baseline with an angle at each end. He has onlyto draw the baseline on the plan, to any convenient scale, andto make similar angles at the two ends. These angles, whenproduced, will meet each other, forming a triangle, which willshow to scale the precise distance and position of the whitestaff, this being represented by the top point or apex of thetriangle. (See Fig. i.) lOI To obtain a complete survey of any district a number ofthese triangulations are made, the sides of the first trianglebeing used as the basehne of others, and so on. Each set ofresults is carefully entered in a notebook as taken, and all arefinally plotted out on the finished plan or map. OBLIECI V. orai^ MOUNTED PRINTS K LENSES OR EYES N\ if: j BACS VIEW 0 F -J EGATIVE THEODOLITE THEODOLITE , Now, there is really a close analogy between the surveyorsoperations and the phenomena of ordinary human vision. Sup-pose we call the distance between our two eyes the basehne,and consider the eyes themselves as theodolites. Let the eyesbe directed to a distant white staff, or any similar object. Whatis it that happens? Obviously, the only thing that can occur,unless we are to see a double image of the staff, is that theeyes must each turn inwards, so that they are both directedtowards the same object. They are, in fact, converged togetheron the white staff in just the same way as two theodoliteswould be, if both were in use simultaneously on one baselineby two separate surveyors. The brain, that marvelous auto-matic surveyor, instantaneously completes its problem of tri-angulation and obtains accordingly a truthful impression ofthe distance and pos
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