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The instrument has a focal length of 54 ft.
But the eye is tolerant of small changes in the focal adjustment which sensibly affect the scale-value.
The tangent at any point bisects the angle between the focal distances of the point, and the normal is equally inclined to the focal distances.
They are placed at a distance apart less than the focal length of a, so that the wires of the micrometer, which must be distinctly seen, are beyond b.
Two tangents from any point are equally inclined to the focal distance of the point.