Eyeball-measuring unit

This is an optical instrument that serves to measure the projection of the eyeball. It consists of a rail on which there are two frames - one fixed and one movable. On each frame has an optical prism and millimeter scale on which you see the projection. The rail has a division for reading the distance of survey and it is taken into account if measured again after time. When studying, this unit is placed so that both side portions are tightly touching the outer edges of the orbits.