https://foxoyo.com/mcq/91939/in-an-external-focusing-tachometer-the-fixed-interval-between-static-hairs-is-5-mm-the-focal-length-of-the-objective-is-25-cm-and-the-distance-of-the-vertical-axis-of-the-instrument-from-the-optical-centre-of-the-objective-is-15-cm-which-one-of-the-following-is-the-set-of-constant-of-the-tachometer