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50 Ophthalmic Lenses

Now construct a rectangle based on this and draws in a diagonal from
the center of the cross lines to the corner of the rectangle as drawn in
Figure 3.12.

Fig. 5.12: Construction completed to show magnitude and angle of oblique prism

This line represents both the magnitude and the base setting of the
resultant prism. The line measures 5 units and therefore equals 5Δ diopters.
The direction in relation to the center of the cross lines and nasal area is
down and out. Expressed in terms of a 360° notation the base direction is
323°.
If a 180° notation is required the diagonal line needs to be extended so
that it lies above the horizontal, measuring the axis indicated as shown in
Figure 5.13. Therefore, the answer to compounding the prisms is:
5Δ base at 143° out
or, 5Δ base at 323°

Fig. 5.13: Showing angle of prism in standard notation