. Machinery and processes of the industrial arts, and apparatus of the exact sciences. BC r. BC cosABC cosABC Hence the distance between the foci of the two lenses must be made tovary directly as the cosine of the angle of inclination to the horizon. A mechanism producing a variation according to this law is illustrated in principle in Fig. 123. Let A B be two fixed points in a horizontal bar, supported by an upright P. Attached Fig- 123, to these two points let there be two arms so pivoted as to turn freely. The arm pivoted at B is to be constructed hollow; and a rod sliding within it is to b
. Machinery and processes of the industrial arts, and apparatus of the exact sciences. BC r. BC cosABC cosABC Hence the distance between the foci of the two lenses must be made tovary directly as the cosine of the angle of inclination to the horizon. A mechanism producing a variation according to this law is illustrated in principle in Fig. 123. Let A B be two fixed points in a horizontal bar, supported by an upright P. Attached Fig- 123, to these two points let there be two arms so pivoted as to turn freely. The arm pivoted at B is to be constructed hollow; and a rod sliding within it is to be pivoted to. TELEMETRICAL APPARATUS. 583 the extremity D of the arm pivoted at A. In consequence of the con-nection thus established, neither of the two arms can turn withoutcausing the other to turn; and with every change of position of thearms there will be a variation of the distance between the points B andD. This distance will reach its maximum when the arm takes the hor-izontal position AO. Suppose now the arm AD to be equal to the dis-tance AB. The triangle ABD is isosceles, and the base BD has thevalue expressed in the equation, BD=2ABcosABD. Now since AB is horizontal, the angle ABD is the angle of inclinationof BD to the horizon; so that if the two lenses, in the case supposed,were fixed so that their focal points should be at the points B and D, thedistance of these points from each other would vary according to therequired law. The manner of adapting a contrivance of this description to a tele-scope, so as to make the instrument fulfill the prescribed condition, isshown in Fig. 124.
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