Fresnell press-on lens used to treat several ocular mobility disorders, during power changes or vision therapy and can be used to treat several ocular motility disorders including strabismus - lens are sold individually - click on the photo for other details - made of polyvinyl chloride, 1mm thick - 63.5 mm diameter - you can apply a Press-On prism to the back surface of one or both lenses of the patient`s eyeglasses with just water - it can be applied to the entire lens or to any region of the lens - units/package: 1