A lens consisting of two cylinders of equal power but of opposite designation, one being plus and the other being minus, and their axes set 90 degrees apart is known as a:

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A lens consisting of two cylinders of equal power but of opposite designation, one being plus and the other being minus, and their axes set 90 degrees apart is known as a cross cylinder. A cross cylinder is used to determine and check the axis of the correcting cylinder as well as to check its power.

A Lens Consisting of Two Cylinders of Equal Power but of Opposite