50 mm diameter with ground and polished faces and ground edges.

The micrometer slit is as above but has a heavier screw thread, 0.5mm pitch x 6mm dia, carrying a drum which is divided into fifty divisions giving a reading 0.01mm. An index is provided with a 5mm scale divided in 0.5mm divisions.

A circular metal screen 75mm dia, with 10mm central aperture covered with 1.5mm mesh wire gauze. Mounted on a 6mm brass rod. The optical centre height is 80mm appгох.