Lens Holder Set, Basic Optics An accessory for PASCO's Basic Optics Systems. Mount 50 mm diameter lenses, mirrors, etc. into the empty lens holder and secure with retaining rings. Holders protect lenses and make them easier to store. Includes 2 lens holders. The Lens Holder Set -- Basic Optics (OS-8522) is an accessory for the: Basic Optics System (OS-8515C)