These very accurately machined cylinders are used to remove the error of perpendicularity of the index mirror of the sextant. By placing the cylinders on the arc of the sextant while laying on its side, and bringing them into conjunction through the optical system, the index mirror can be adjusted to make it perfectly perpendicular to the arc.

Sextants that have front surface silvering on the index mirror benefit most from these cylinders. This is because the silvering is not exactly on the axis of rotation, and requires that your line of sight be parallel to the sextant frame when adjusting the index mirror.

The cylinders come in a velour pouch and can be readily carried in the sextant case.

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