iris diaphragm

 An adjustable diaphragm, reminiscent of the iris of the eye in its action, by which the diameter of an approximately circular opening may be controlled, as for regulating the aperture of a lens; it consists of a number of movable thin curved plates fastened at regular intervals around the inside of a ring, the positions of which are simultaneously adjusted by a single knob on the outside of the lens. It is used in cameras and microscopes.  [Webster 1913 Suppl.]