Lectures on Practical Astronomy and Astronomical Instruments (1879)
Challis, James Rev. · Deighton, Bell and Co. (Cambridge) · leaf 398
le the mutual inclination of the planes of the reflectors. The arc BC is so graduated that this double angle is immediately read off by means of the vernier V, which is connected by a bar with the index-glass J, and turns with it about an axis passing through the centre of the graduation. The… … that direction, and 2° from zero in the opposite direction, and each graduation-interval is 20’. As 39 graduation-intervals are equal to 40 vernier-intervals, it follows that the arc can be read off to 30", and by estimation to a fraction of that quantity. {In Troughton’s sextants, the…
Book · Astronomy
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