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A NUMBER of years ago, while checking segment position on a patient who was having "bifocal trouble," I noticed that a flashlight held in front of the glasses cast a sharp linear shadow on the eyeballs and lower lids.
If the ophthalmoscope light in a semidarkened room is held at a distance of 33 to 45 cm. directly in front of the eye on the same level as the pupil, the position of the segment top is shown by the line appearing on the eyeball (Figure, upper left). The proper position differs with the individual subject, of course, but should be such that the segment is not in the line of vision (with the head held in habitual position) for distant vision (Figure, upper right) and the top segment line is not in the line of vision for reading (Figure, middle left), and the segments should be the same distance
ALLEN DG. METHOD OF CHECKING SEGMENT POSITION. AMA Arch Ophthalmol. 1954;52(3):456-457. doi:10.1001/archopht.1954.00920050458015