With the patterns horizontal (the control handle in a vertical position) when the cornea is spherical, or has with-the-rule toricity, the two cross patterns will appear to be on the same line (a). To make the dioptric measurement the central screw in the control handle is rotated until the central solid cross is inside and aligned with the central hollow cross (b). The dioptric value is then read from the middle window (2) in the instrument (the right window gives the reading in millimeters (3)).
The instrument can read dioptric values from 25.50 to 83.00 D.
