The National 799-155 100x Retractable Oil Immersion Objective Lens is for use with the 155 microscope series. It features a 1.25 numerical aperture and 100x magnification. A retractable objective lens provides additional protection against lens or slide damage. The lens is also sealed within the objective casing, allowing the end of the objective to be immersed in oil during use without letting oil into the lens itself.