Nikon CF achromat 40X microscope objective for the YS-100 Nikon microscope. It is 160mm corrected with an na of 0.65, working distance of 0.6mm and requires a cover slip thickness of 0.17mm. It probably will work with the Nikon alphaphot-2 YS also. The Nikon number is MSB50400. Our company fairly represents the condition of products listed and stand behind the products sold.