(under subclass 642) Subject matter wherein the components of the lens are so shaped and spaced along the optical axis of the lens that the total refracting power of the lens is zero, i.e., parallel light rays, upon entering the lens and passing through all components thereof are so refracted that they emerge from the lens as parallel rays.
(1) Note. These lenses may be attachments to be used with other lenses to change the equivalent focal lengths of the latter lenses.
