Contact lenses have changed dramatically since the first glass contacts were invented in Switzerland in 1888. Modern lenses are made of several different materials, each with advantages and disadvantages:

* Hard lenses, made of a plastic known as polymethylmethacrylate (PMMA), are rigid, small, light-weight and safe. Impermeability to oxygen, which intereferes with the flow of oxygen to the cornea, is the major disadvantage. This limits daily wearing time to eight to 16 hours. They are also the most difficult kind to wear, and users experience a blurring when they switch from lenses to glasses.

* Soft lenses, made from one of several water-absorbing plastics called "hydrogels" or from soft silicone, absorb water, are easier to adapt to and can be worn for longer periods than hard contacts. But they also are fragile, provide less acute vision correction, and may increase the likelihood of eye infections because they become contaminated from handling and wearing. Soft lenses can be worn on a daily basis or for extended periods, depending on their type.

* Gas-permeable lenses have the superior optical and ease-of-care qualities of hard lenses because of their rigidity but have the comfort of soft lenses because of their permeability.