disc micrometer
(US disk micrometer)
noun
1 Astronomy a micrometer in which the image in a telescope or microscope is made to fall on a circular disc, placed at such a position as to match the image in diameter, the position and diameter of the disc then allowing the angular size of the image to be calculated; now rare and historical.
2 Engineering a micrometer in which the two jaws end in a circular disc to provide a larger grasping area.
Origin
Late 19th century; earliest use found in American Journal of Science.