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575.—The Omnimeter is one of the class of instruments in which the tangent to a radius proceeding direct from the axis of the telescope is represented by the stadium made of constant length, the subtense angle varying with the distance. The omnimeter is the invention of Chas. A. C. Eckhold, a German engineer, described in the provisional British patent[32] as "a person living in Alexandria." The instrument as originally devised consisted of a kind of theodolite to which the subtense tangential system was added as an entirely separate part. The important part of the provisional specification shows that the principle of the invention consists in the use of two sights to the instrument, one a telescope to sight the object, and the other a powerful compound microscope to read divisions upon a tangential scale. The telescope and microscope are firmly united together in parallel position with their axes exactly crossing the transverse axis of the theodolite, so as to move together through the same angle by the motion of the telescope in traversing the azimuth. A delicate level is placed upon the telescope, and when the bubble is in the centre of its run the scale is truly at right angles to the axis of the microscope. The scale in the early instruments stood vertically at the extreme edge of the instrument in a position lateral to the object-glass of the telescope. It was finely divided to millimetres, and read the intervals of the divisions by means of a micrometer screw with a vernier.
Fig. 251.—Details of omnimeter, showing section of microscope and scale.
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576.—With the instrument as originally constructed, it was found that the delicate scale, protruding vertically to the extreme edge of the instrument, was very liable to injury unless supported by heavy metal work, which rendered the instrument cumbersome. A great improvement was made in this instrument, which brought it to its modern form, by placing the tangent scale in a horizontal position, where it could be firmly fixed upon the vernier plate as shown Fig. 251 S, and reading the scale by means of a reflecting prism P in the eye-piece of the microscope. In this improved instrument, as the microscope and telescope are still united on one axis so that they move at equal angles to each other, it is clearly indifferent whether the scale be placed vertically or horizontally, provided it is placed truly at right angles to the microscope when the axis of the telescope is horizontal. The scale, which is 4 inches long, is placed in a sliding fitting to adjust longitudinally to its position by means of a micrometer screw. In the English instrument the scale is divided into 100 parts for calculation. The divisions are subdivided by shorter lines, making the actual division 200. The micrometer screw has 50 threads to the inch, and moves over one of the divisions of the scale only. The micrometer head is divided into 100, numbered at the tens; a vernier placed against the head subdivides each of these divisions into 5, making the total micrometer 500 for one complete revolution. The total division of the 4-inch scale therefore becomes: 200 (divisions of scale) × 500 (micrometer) = 100,000 in 4 inches. The scale is placed centrally to the instrument, so that when the telescope is level the microscope is vertical, and reads 50,000 when in perfect adjustment.
Fig. 252.—Details of prismatic eye-piece.
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