horizon glass

The second mirror, referred to as the "horizon glass", is fixed to the front of the frame.

One half of the horizon glass is silvered and the other half is clear.

Ata took the wheel and adjusted the horizon glass of her sextant; she looked impassive and ethereal.

Light from the celestial body strikes the index mirror and is reflected to the silvered portion of the horizon glass, then back to the observer's eye through the telescope.

Mirrors, called the index glass and the horizon glass, are fixed one upon the index arm and the other upon one side of the frame, respectively.

The observer manipulates the index arm so the reflected image of the body in the horizon glass is just resting on the visual horizon, seen through the clear side of the horizon glass.