Many forms of glasses have been devised to enable persons to see at a distance with one part of the lens and at a close range with the other part.
The bifocal is usually placed in the lower segment, although sometimes it is in the upper segment or in the center. These various arrangements of lenses afford either long or short distant vision through only a part of the lens, and it is impossible to view the whole field in all directions.
The spectacles recently devised consist of two pairs of lenses in two separately hinged frames. The first pair gives full vision for distance. When the second pair is lowered against the first pair, it supplies the additional lenses necessary for full near vision. For certain work, the full vision is desirable or even necessary.
For the ear, nose and throat specialist whose offices are usually made up of
Metzenbaum M. FULL VISION BIFOCAL OPERATING SPECTACLES. JAMA. 1926;86(3):193. doi:10.1001/jama.1926.26720290001014a
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