*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. +PJC ]