This instrument has tribrach levelling with quick-setting spherical lower plate, a sliding tribrach for centring over any desired spot, and full clamp and tangent motions to both horizontal centres. The dividing is upon silver on a 6-inch covered limb reading by two verniers to single minutes, folding sights interchangeable with telescope Y's, and this dial may be used upon any staging without its stand. The somewhat peculiar shape of the cranked rocking ring is necessitated by the movement of the sliding tribrach, which it has to clear in all positions for reading vertical sights.
530.—Accessories Common to Hedley's Dials are a vertical reflector and a diaphragm illuminator.
Reflecting Cap.—One of the disadvantages of Hedley's dials over Lean's was pointed out to be the impossibility of vertical sight where the two last described dials are not used. Some years ago the author devised a plan of obtaining this vertical sight by reflection by means of a reflecting cap, Fig. 220, placed over the end of the telescope. The cap is formed of a tube which fits the outer surface of the object end of the telescope. This is prolonged sufficiently to lock it by a dowel in correct position against revolution when the points that are used for index in the diaphragm of the telescope are vertical. The tube is cut in two and hinged to turn up, as shown in two positions H and H′. When turned up it leaves the tube open for direct vision. A reflector R is placed in the cap, and there is an opening below it equal to the full aperture of the telescope. It is easy to see that by this means a pair of lights or a line may be sighted up or down a shaft, and the azimuth of its direction be reflected to follow a line by slightly rocking the telescope upon its pivots. This may be done, however, with more refinement if there is a clamp and tangent motion to the vertical arc, which is placed only on first-class instruments.
Fig. 220.—Reflecting cap to miner's dial.
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531.—Illumination of the Diaphragm for observing the webs or a point, may be conveniently effected underground by employing a conical ring reflector in front of the object-glass. The aperture through the cone leaves the field of the object-glass nearly free, as it is only necessary that the cone should project in front of this for a very small distance. This reflector is placed over the object end of the telescope when it is required, just the same as the ray shade. The vertical reflector, Fig. 220, goes on the same fitting. The reflector Fig. 221 R may be made of silver or platinum. A light placed anywhere opposite this, and perpendicular to the axis of the telescope will throw sufficient light to show the webs or point. Sometimes a simple, plain mirror placed on an arm bent over to the centre of the front of the object glass, in which the mirror stands at 45° to the axis, is used; but this plan is not so good as that shown Fig. 221, as the light has to be brought to face the mirror quite perpendicular to the axis of the telescope, and this process is frequently difficult to accomplish underground.
Fig. 221.—Conical reflector to illuminate axis of telescope.
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