Fig. 242.—Stanley's patent subtense diaphragm.
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560.—The Subtense-diaphragm of the author, Fig. 242, forms the eye-piece of a theodolite. It has movable indices which are separated according to a scale formed by calculation upon the data of the above formulæ. By this, distances may be taken in the horizontal plane for land of any inclination without after calculation. This result is obtained by observing the angle of inclination upwards or downwards on the theodolite and setting the micrometer to this angle before reading the subtense distance. The reading is taken by points which are arranged to measure the subtense 1 to 100, so that the ordinary Sopwith staff may be used. The diaphragm at zero appears as an ordinary subtense-diaphragm. It may be observed that this diaphragm may be used as a good check, as distances may be taken over any irregularities of intervening incline and give the true base for the entire distance.
561.—If the mean contour distance is required from station to station, this may be taken directly by subtense from the staff-reading held at right angles to the axis of the telescope. The means of doing this, devised by the author, is to place a sight director of a special form upon the side of the staff, Fig. 243. This small piece of apparatus is shown attached to the staff. It consists of a small telescope three inches long attached at right angles to the staff by means of a dovetail slide fitting when in use as shown. The staff-holder sights the tacheometer through the short telescope, which can only be seen to appear therein by moving the staff until it is approximately at right angles to the direction of the tacheometer.
Fig. 243.—Sight director for stadium.
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562.—The Anallatic Telescope.—In this telescope the focus is constant, and consequently the tangential measurements indicated by the numerical qualities subtended by a constant angle are directly proportional to the base, so that there is no constant to be added. The invention of this instrument and its modern application to subtense measurement was due to Professor J. Porro, of Milan, who put it to practical test in 1823,[29] in an instrument termed a tacheometer. The telescope will be best understood by the following details:—
The object-glass O, Fig. 244, is made of a focus that falls well in front of the axis of the instrument CC′, so that the rays cross before falling upon the anallatic lens A, the optical arrangement being such that if the rays fell direct without any refraction they would reach the axis and subtend angles therefrom inversely proportional to the distance of the stadium. The object-glass and anallatic lens are of the same focus, so that the rays after crossing from equal refraction may emerge parallel in the space A to M. The stop at S and at the axis CC′ cuts off eccentric rays that would otherwise give internal reflections from the telescope tube. The eye-piece, represented by MF, may be made to pick up the image of the stadium in front of it upon an ordinary webbed diaphragm or upon ruled glass. The diaphragm webs are fixed, or the glass surface engraved with three or five horizontal lines and one vertical. The outer horizontal lines are used generally as the subtense lines, and the central line for levelling and taking altitudes. The vertical line is used for triangulating on the surface of the ground.