Comstock's Method

a method for testing the quality of the level in a theodolite and determining the value of its scale divisions. It was proposed in 1885 by the American astronomer G. Comstock (1855–1934).

Comstock’s method makes it possible to test the level in the field, without removing it from the instrument. The instrument is carefully leveled, and its vertical axis is tilted through a known angle I ≈ 1° by adjusting one of the supporting screws. The bubble of the level being tested then deviates from the middle of the scale. By rotating the instrument about its vertical axis until the axis of the level becomes parallel to the line of intersection of the plane of the dial with the horizon, the bubble may be returned to the middle. If the instrument is then rotated in both directions relative to this position, the value of a scale division of the level is determined by the formula

τ” = 1.0472 · I⁰β’/(i₂ ˗ i₁)

where (i₂ — i₁) is the shift of the bubble in scale divisions when the instrument is rotated through a small angle β’ (measured in minutes of arc), which is measured on the graduated circle.