a lens or mirror that produces a virtual image of an object smaller than the object itself.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
Examples from the Web for reducing glass
The reducing-glass will help you to get the size and position of things.The Painter in Oil|Daniel Burleigh Parkhurst
British Dictionary definitions for reducing glass
a lens or curved mirror that produces an image smaller than the object observed
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