<P> Original bifocals were designed with the most convex lenses (for close viewing) in the lower half of the frame and the least convex lenses on the upper . Up until the beginning of the 20th century two separate lenses were cut in half and combined together in the rim of the frame . The mounting of two half lenses into a single frame led to a number of early complications and rendered such spectacles quite fragile . A method for fusing the sections of the lenses together was developed by Louis de Wecker at the end of the 19th century and patented by Dr. John L. Borsch, Jr. in 1908 . Today most bifocals are created by molding a reading segment into a primary lens and are available with the reading segments in a variety of shapes and sizes. The most popular is the D - segment, 28 mm wide (citation needed). While the D - segment bifocal offers superior optics, an increasing number of people opt for progressive bifocal lenses . </P> <P> Bifocals can cause headaches and even dizziness in some users . Acclimation to the small field of view offered by the reading segment of bifocals can take some time, as the user learns to move either the head or the reading material rather than the eyes . Computer monitors are generally placed directly in front of users and can lead to muscle fatigue due to the unusual straight and constant movement of the head . This trouble is mitigated by the use of trifocal lenses or by the use of monofocal lenses for computer users . </P> <P> In an interesting legal case reported in the UK in 1969, the plaintiff's ability to use bifocals was impaired by accident . </P> <P> Research continues in an attempt to eliminate the limited field of vision in current bifocals . New materials and technologies may provide a method which can selectively adjust the optical power of a lens . Researchers have constructed such a lens using a liquid crystal layer sandwiched between two glass substrates . </P>

The upper and lower portion in bifocal lenses