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Bifocal Lenses 125

segment top in case of first time wearers. However, if the bifocal prescribed
is mainly for near vision, then the segment top might be fitted little higher,
say, midway between the lower edge of the iris and the lower edge of the
pupil (Fig. 10.24B). If the lenses have been prescribed for some vocational
purpose and are to be designed for only occasional near vision use, then
the segment top might be fitted 2-3 mm lower than the normal (Fig. 10.24C)
These suggested positions of the segment top assume that the head is held
in the subjects primary position with the eyes viewing a distant object. But
if a person who habitually holds his head high will need a lower segment
height than a person who slouches a bit and keeps his head downward. A
tall person who habitually keeps his head down may need a little higher
segment top whereas a short height person who has to keep his head up
may prefer a little down segment top. Past wearing habit of the subject
should also be given importance while positioning the segment top. If a
wearer is used to wearing bifocal segment up, fitting a lower segment in
the new spectacle will create trouble for him to adjust immediately.

Figs 10.24A to C
Various kinds of dispensing aids have been proposed from time to time
to assist in the fitting of bifocal lenses, but most practitioners obtain excellent
and consistent results by simply measuring the segment height with a
millimeter ruler. Ideally the segment height should be converted into a
segment top position in relation to the horizontal centre line of the frame.
A typical routine procedure for taking the measurement is:
1. Choose the final frame and adjust it to fit the subject correctly (Fig. 10.25).

Fig. 10.25