Rack the condenser down slightly and cut down the aperture of the iris diaphragm so that the light, although even, is dim.
"The Elements of Bacteriological Technique" by John William Henry Eyre
Diaphragm carrier with iris diaphragm.
"Microscopes and Accessory Apparatus" by Ernst Leitz
An iris diaphragm regulates the intensity of incident light.
"Life Movements in Plants, Volume II, 1919" by Sir Jagadis Chunder Bose
Hence, for iris photometry of a saturated stellar image of radius of 10 arc sec or more, when the star is centred in the iris diaphragm, there is a linear relationship between the logarithm of the measured transmission and magnitude5 .
Aperture Photometry of Saturated Star Images from Digitised Photographic Plates
Therefore the cutting screen can be like an Iris diaphragm (f-stop) in a photocamera, which cuts only the board of the torus, leaving the central part free -.
New Revelation of Lightning Ball Observation and Proposal for a Nuclear Reactor Fusion Experiment