This set is one of the earlier series of Olympus combination Brightfield / Darkfield microscope objectives for metallurgical / semiconductor observation. They have a 25mm diameter thread in order to accommodate the annular condenser assembly concentric with the central objective. These objectives should only be used with the 'Neo' type nosepiece - the standard nosepiece has an RMS thread which is closer to 20mm. Objectives are also unsuited for use with the later style infinity tubelength vertical illuminators. Pictures show actual items.