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Eccentricity of the Sextant by Frederic Furbish, 1893

Eccentricity of the Sextant by Frederic Furbish, 1893, Page 65

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This can be very accurately done, for the images are usually, unless the weather be hazy or misty, very sharp and distinct. Read the Vernier and in reading it be careful to bring the reading glass directly over the mark of coincidence. Record the reading in the column marked "on arc" for we will assume the reading to be on arc. Now unclamp the Vernier and move the index arm so that the lower limb of the reflected image (b') and the upper limb of the real image (a) (as seen in the field) are near. Clamp the Vernier and bring them into the position of tangency with the tangent screw. Read the Vernier and record the reading in the column headed "off arc", for if the first reading was "on arc" the second will be "off arc", the third "on arc" again and so on through the whole set.
 
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