Direction of the line from apex to base in a principal section of a prism. NOTE1: The setting position for the prism base may be indicated by the directions 'base up' or 'base down' or 'in' (towards the nose) or 'out' (towards the temples). The base setting may also be indicated by the angular coordinate system specified in ISO 8429. NOTE2: The base setting of a lens is the direction in which all rays passing through the lens are uniformly deviated, any focal power having been neutralized.
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