substage

The shape of the aperture is a not unimportant point; it should be square to allow of free movement over the substage condenser.
The Elements of Bacteriological Technique|John William Henry Eyre
If it no longer occupies the centre of the field, alter the angle of the substage mirror until it does.
The Elements of Bacteriological Technique|John William Henry Eyre
Remove the substage condenser and replace it by the dark ground condenser.
The Elements of Bacteriological Technique|John William Henry Eyre
The lower substage (Clevedonian) is well displayed near Clevedon in Somerset.
Encyclopaedia Britannica, 11th Edition, Volume 3, Part 1, Slice 1|Various

Adjust the substage mirror so that the light is reflected upward.
The Elements of Bacteriological Technique|John William Henry Eyre

British Dictionary definitions for substage

substage

/ (ˈsʌbˌsteɪdʒ) /

noun

the part of a microscope below the stage, usually consisting of an adjustable assembly holding a condenser lens for illuminating the specimen

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012