jump

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jump

The displacement of the image of an object occurring when viewing across the borderline between two portions of different power in a bifocal or trifocal lens. The jump is eliminated by placing the optical centres on the dividing line; the lens is then called a no jump bifocal (e.g. a monocentric bifocal with a straight dividing line) (Fig. J1). Syn. image jump; prismatic jump. See monocentric.
Fig. J1 Image jump upward caused by the segment of a bifocal lens, as the direction of gaze is lowered across the dividing lineenlarge picture
Fig. J1 Image jump upward caused by the segment of a bifocal lens, as the direction of gaze is lowered across the dividing line
References in periodicals archive ?
Occasionally we would realize a cow or horse had jumped the rails, and my cowboy-husband would rope them in before the police did.
According to a JR study, the train should not have jumped the rails unless it was moving at 133 kph or faster.
The 74-year-old's first brush with death came in 1962 when a train he was travelling on from Sarajevo to Dubrovnik jumped the rails and plunged into an icy river.