... 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