. Machinery and processes of the industrial arts, and apparatus of the exact sciences. ent object, O, will be bent in such a manner as tofall into the direction OE; and if OAW be equal to the angle of parallaxin any observation, say ABA7 in Fig. 131, the system ABOD, inter-posed between the eye and the prism B, will make the direction of the dis-tant object coincide with BA, or interposed between the eye and the spaceabove the prism through which A7 is seen in the same figure, it wouldmake A appear in the direction EB; only that, in this case, it wouldbe necessary to displace the convex band i
. Machinery and processes of the industrial arts, and apparatus of the exact sciences. ent object, O, will be bent in such a manner as tofall into the direction OE; and if OAW be equal to the angle of parallaxin any observation, say ABA7 in Fig. 131, the system ABOD, inter-posed between the eye and the prism B, will make the direction of the dis-tant object coincide with BA, or interposed between the eye and the spaceabove the prism through which A7 is seen in the same figure, it wouldmake A appear in the direction EB; only that, in this case, it wouldbe necessary to displace the convex band in the direction opposite tothat shown in Fig. 134, or toward the left. This system of lenticular zones is introduced into the apparatus onthe right, Fig. 130, at the point marked L. The concave band is fixed,and the eye always looks through its optical center. The convex bandis movable, and is displaced by the observer by means of a milled deviations of the ray produced by displacement are sensibly pro-portional to the displacements themselves, or to the distances of which. o 0/ / b IV—— -——-^^ ?nh-- \ ——Ipt* jji -v ? ??? u 592 PAEIS UNIVERSAL EXPOSITION. they are the parallaxes. These distances may, therefore, be marked onthe scale which indicates the amount of displacement, when the lengthof the base line is fixed. Observations with this apparatus may be very rapidly made, and theirfacility is such that it is usual to take as many as ten successively, andto adopt the mean result. With a base of twenty meters, the meanerror for distances up to one kilometer is one-half of one per cent.; fordistances as great as two kilometers or upward, two per cent., increas-ing as the square of the distance. By using a base twice as large—forty meters instead of twenty—the errors are reduced to one-half theforegoing. These determinations are made without a telescope. Whilea telescope may increase the accuracy of the results, it sensibly dimin-ishes the facility of ob
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