Micrometer Eyepiece, by Simms, London, 19th Century

Instrument is circular in design with a scale around the rim marking 10mm increments. Instrument has a central hole which is covered by a central bar. Two rotating devices either end of the bar control the size of the central aperture. Aperture is covered by a removable dust cap. A dial on top of the device rotates a separate Vernier scale held underneath main circular scale.