... from none other than Sir David Brewster, 1853:
Lay a piece of clean glass flat and level on a surface.
Pour just enough skim/1% milk on the surface of the glass to cover it completely.
Let it dry!
(spray it with matte fixative afterwards to prevent "deterioration" of your focusing surface!).
– from Primitive Photography by Alan Greene, Focal Press, 2002 ISBN 0-240-80461-9