Metal screen, with 12.5mm central aperture. A screw controlled, spring loaded precision slit is mounted on one side of the disc, the adjustment drive being arranged so that the slit can not be over tightened. The slit has levelled jaws with brass, dovetail section slides, & provides a maximum slit opening of 4mm approx. with a useful length of 10 to 12mm. Mounted on a standard 6mm diameter brass rod, the optical centre height is 80mm approx.