In the mineral descriptions found in the text book the terms LENGTH FAST and LENGTH SLOW are encountered.
Length fast means that the fast ray of the mineral vibrates parallel with the length of the elongate mineral or parallel to the singel cleavage, if present. This is also referred to as NEGATIVE ELONGATION, as the overall total retardation is less than that exhibited by the mineral prior to the accessory plate being inserted.
Length slow means that the slow ray of the mineral vibrates parallel with the length of the mineral or the single cleavage, if present - POSITIVE ELONGATION, the total overall retardation is greater than that exhibited prior to the accessory plate being inserted.
Only minerals which have an elongate habit exhibit a sign of elongation.