If you need a spare for safety or wish to use and try different working distances these prisms combined with the Tech-Line Binocular Headband Magnifier are a good investment. Please choose your magnification below. Stated price is for one prism.
Details on Magnification and Working/Focal Distance and Optics above.
The optics are made from silicate glass, which is very scratch-resistant, has a very high resolution (very sharp!) and produces an extremely high contrast image- Schweizer uses a UV absorbent optical crown as used with prescription lenses (refractive index n=1.52).