Fig. 76.
Fig. 77.
Difference of reading with a web and a point, shown much magnified..
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238.—In early levels of improved construction, as shown [Fig. 70], a difficulty was experienced in practice in bringing the index point exactly up to the edge of the line as it is shown in Fig. 77 at P. This difficulty has been obviated in recent highest class instruments by making a tangent screw adjustment to the axis as shown under the level in Fig. 78. There was a great objection to the old form of tangent adjustment by clamping on the axis, as this was found to disturb the centre. In the plan shown in the illustration the clamp is left free by jointing to the axis until it grips one of the arms of the tribrach upon a vertical surface; in this way it cannot disturb the axis. The level, Fig. 78, is shown mounted on a framed stand, which is preferred by the Indian Government, and is generally necessary for rigidity for large instruments of over fourteen inches. This will be described further on with theodolites, art. 447, on framed stands.
Fig. 78.—Stanley's improved dumpy.
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239.—Stadia Points.—The author commonly makes the points, Fig. 75, VV′ stadia points, by making the distance of the extreme ends of these subtend an angle, equal to 10 feet of the levelling staff at ten chains distance, or 1 foot of the levelling staff at 100 feet distance (+ a constant to be discussed Chapter XII.), by which measurements of the distance of the staff can by taken or checked by observation through the telescope only.