A lens mounted in or below the stage whose purpose is to focus or condense the light onto the specimen.   The higher power objective lenses have very tiny diameters and require concentrated light to work properly.  By using a condenser lens you will increase the Illumination and resolution.  Condenser lenses are not required on low power microscope [..]

A positive lens or set of lenses designed to collect light and project it evenly across an area. Commonly found in projector systems as part of the condenser lens portion.