Abstract: The cross section of a beam of light may be controlled by three spacedly superimposed circular plates formed with respective, central apertures circular about the axis of the passage defined by the aligned apertures. The central plate defines with the two outer plates respective gaps in each of which two slides may be moved toward each other into a position in which they jointly block the passage, the paths of the two pairs of slides being perpendicular to each other. Springs hold the stack of plates and slides together so that the slides are guided by the plates bounding the corresponding gap, but thermal expansion of the elements of the stack does not interfere with the mobility of the slides which may be adjusted manually.