Also vernier scale. a small, movable, graduated scale running parallel to the fixed graduated scale of a sextant, theodolite, barometer, etc., and used for measuring a fractional part of one of the divisions of the fixed scale.
Machinery. an auxiliary device for giving a piece of apparatus a higher adjustment accuracy.
French mathematician and maker of scientific instruments, known especially for his invention of an auxiliary scale (named after him) used for obtaining a highly precise reading of a subdivision of an ordinary scale.