Leitz Phaco 40X Phase Contrast Microscope Objective for 170mm TL Leitz microscopes of he 1960's, 70's and early 80's. This objective has an na of 0.65 and requires a a coverglass thickness of 0.17mm.. It has been tested and found to be optically perfect with no lens delamination, fungus, dust, or scratches. You won't be sorry if you purchase this lens! Thanks for looking.