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Astigmatic Lenses 35

used for the axis of cylinder, the full 360° protractor is only required for the
base direction of prism. When describing a horizontal cylindrical axis, it is
conventional to use the angle 180°, rather than zero. The orientation of
cylindrical axis is opposite the dispenser’s side as compared with viewing
the axis from the subject’s side (Figs 4.3 to 4.5).

Fig. 4.5: Orientation of cylindrical axes on a protractor

A good way to recognize cylindrical axes from dispenser’s side is to
view the back of your left hand; the thumb will point to the zero degree
(Fig. 4.6).

Fig. 4.6: Thumb will point to zero degrees

TORIC LENSES
Toric lenses, also known as sphero-cylinder lenses can be explained as
spherical lens that has been placed in contact with a plano-cylinder lens
(Fig. 4.7).