| 摘要 |
A vertical post extends through the center of an enlarged central opening in an annular circular disc. Manually adjustable mechanisms removably secure the disc to the post in such manner as to permit the angle between the plane of the disc and the axis of the post to be set at any value between -X and X, where X as expressed in degrees can range between 0 and somewhat less than 180. A first elongated vertical member with a semicircularly shaped central section has upper and lower horizontally elongated hollow tracks extending in the same direction and coplanar with the section. The post passes through the tracks and section with clearance so that the member can be moved vertically up and down with respect to the post and also can be rotated around the post. A second elongated vertical member has a straight vertical central section provided with a guide mechanism engageable with an disengageable from the periphery of the disc. The second member has parallel horizontal upper and lower rails at corresponding upper and lower ends of the straight section. Each of the rails slidably engages a corresponding one of the tracks and is horizontally movable therein. A marker device slidably engages the lower rail and is securable thereto.
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