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Prisms 49

Fig. 5.9: Standard notation

Draw a set of lines at right angles to one another and mark the nasal
area along with the primary directions based on this.

Fig. 5.10: Left eye representation showing principal directions

Picking a suitable scale, i.e., 1 cm = 1 diopter, mark a point equivalent to 3Δ
down and 4Δ out.

Fig. 5.11: Shows 3Δ down and 4Δ out to scale