an instrumentality invented for a particular purpose

an optical device for focussing a camera or other instrument automatically

an optical device for obtaining interference fringes

an optical device consisting of an attachment that enables an observer to view simultaneously the image and a drawing surface for sketching it

optical device used to follow the path of a celestial body and reflect its light into a telescope; has a movable and a fixed mirror

optical device consisting of a tube containing a convex achromatic lens at one end and a slit at the other with the slit at the focus of the lens; light rays leave the slit as a parallel beam