from The Century Dictionary and Cyclopedia
- n. A piece of brass oṙ other metal, with a hole or slit in it, attached to a quadrant, azimuth compass, or other instrument, through which aperture the observation is made. See cut under prismatic.
Sorry, no etymologies found.
It consists practically of an ordinary compass box with a prism and sight-hole at one side, and a corresponding sight-vane on the opposite side.
When being used it is held horizontally in the left hand with the prism turned up in the position shown, and the sight-vane raised.