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52 Ophthalmic Lenses

DETECTION OF PRISM IN AN OPTICAL LENS
Hold the glass up between the eye and an object which forms a straight
line. If the continuity of the line in broken as seen in Figure 5.14, it implies
that the prism is present and since the line appears to be deviated towards
the apex, we know the direction of apex of the prism. The amount of
displacement produced is the strength of the prism, and can be measured
by neutralising. To do so put the prism lens of known strength in contact
with the lens under checking with its apex in opposite direction. The prism
lens with which the continuity of the lens is again established, represents
the strength of the prism.

Fig. 5.14

Fig. 5.15: Detection of prism in a lens using lensometer showing 2.5Δ prism