 Though all the above contact lens types—sclerals, PMMAs and RGPs—could be correctly referred to as "rigid" or "hard", the latter term is now used to the original PMMAs, which are still occasionally fitted and worn, whereas "rigid" is a generic term for all these lens types; thus hard lenses (PMMAs) are a subset of rigid contact lenses.Occasionally, the term "gas permeable" is used to describe RGPs, which is somewhat misleading as soft contact lenses are also gas permeable in that they allow oxygen to get through to the ocular surface.The principal breakthrough in soft lenses was made by Czech chemists Otto Wichterle and Drahoslav Lím, who published their work ""Hydrophilic gels for biological use"" in the journal "Nature" in 1959.