Just now I was looking up "azimuth and alidade", and encountered word "vernier".
I looked that up, and was surprised to find it is an eponym:
vernier
n.
after Pierre Vernier (1580-1637), Fr mathematician who invented it6
a short graduated scale that slides along a longer graduated instrument
and is used to indicate fractional parts of divisions, as in a micrometer:
also vernier scale
2 any device that makes possible a finer setting of a tool or measuring instrument
adj.