This device attaches to the bottom of the main optical tube above the objective. Its purpose is to regulate the diameter of the light beam entering the optical tube. This is of critical importance in dark-ground or 'dark field' microscopy. It was therefore most commonly used in conjunction with a dark ground (darkfield) condenser. The lever regulates the size of the opening in the iris diaphragm. In later microscopes a 'funnel stop' which dropped into the rear of the objective, served this purpose and was supplied with the dark ground condenser. In the most expensive modern objectives designed for dark ground illumnation, an iris diaphragm is built into the objective itself.