object glass

from The Century Dictionary.

• noun In a telescope or microscope, the lens which first receives the rays of light coming directly from the object, and collects them into a focus, where they form an image which is viewed through the eyepiece.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the lens or system of lenses in a telescope or microscope that is nearest the object being viewed