Hertel, Christian Gottlieb (1683-1743).
Christian Gottlieb Hertel Vollstandige Anweisung zum Glas-Schleiffen : wie auch zu verfertigung derer optischen Machinen, die aus geschliffenen Glasern zubereitet und zusammengesetzt werden, nebst einer Vorrede Herrn Christian Wolffes.
Halle im Magdeburgischen: Zufinden in der Rengerischen Buchhnandlung, 1716.
"In an attempt to improve image quality for both transparent and opaque objects, two innovations became popular during the eighteenth-century. They both functioned on the principle of reflection. Reflection occurs when light falling upon a surface is turned back. The angle at which the light strikes a smooth surface, the angle of incidence, when measured from the normal, always equals the angle at which the light is reflected in the opposite direction, the angle of reflection. This principle operates in the functioning of a substage mirror on a compound microscope to illuminate transparent objects.
About, 1712, the German, Christian Gottlieb Hertel fitted a plane mirror underneath the stage. This innovation overcame the difficulty of holding the instrument up to the light, and it was rapidly adopted by all instrumentmakers"
(18th-Century Microscopes: A Synopsis of History and Workbook. McCormick).