Graticule: a Scale Traced Through Wax and Etched into Glass with Hydrofluoric Acid

Scales on thermometers were made like this (Thorpe, 1912): coat the glass, trace the divisions through the coat with a fine point, and let hydrofluoric acid bite the bare lines. Acid vapour leaves the lines white and opaque, the liquid acid leaves them clear. On a small glass disc the result is a graticule for an eyepiece. The picture: an eyepiece micrometer scale, Encyclopaedia Britannica 1911 (Wikimedia Commons, public domain).