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 ?
He changed a signal to red as one of the trains overshot a station and jumped the rails.
A well-known trainer, in his riding days, fell on the far side at Chepstow in very thick fog, and believing the race was over, remounted and jumped the rails back on to the racecourse.
One damaged four miles of track near Northampton, apparently because the driver did not realise that two wagons had jumped the rails.