Adjustable Slit Circular blacked metal screen, 75 mm diameter with 12.5 mm 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 4 mm approx. with a useful length of 10 to 12 mm. Mounted on a standard 6 mm diameter brass rod, the optical centre height is 80 mm approx..