Some minerals have elongate habits (e.g., prisms, blades or acicular crystals).  In uniaxial (hexagonal and tetragonal) and orthorhombic minerals (which have parallel extinction), or other minerals with small extinction angles, the accessory plates can be used to determine whether the fast or slow ray is parallel to the long direction of the grain.  If the slow ray is parallel to the length of an elongate mineral, it is termed length slow, or said to have positive elongation.  If the fast ray is parallel to the length of the mineral, it is called length fast, or has negative elongation.  The sign of elongation is determined while viewing a mineral under crossed nicols, with the length of the mineral at approximately 45 degrees to the polarizer and analyzer.  An accessory plate is inserted  and changes in interference colour in the mineral are noted.  If the slow ray of the mineral is oriented parallel to the slow ray of the plate, the interference colours increase by the amount of retardation indicated by the accessory plate.  If the fast ray of the mineral is parallel to the slow ray of the accessory plate, the colours decrease by the same amount.  These changes are termed addition and subtraction.  In A, many of the prisms of sillimanite have colours near first order red.  In B, an accessory plate with retardation of 530 mµ, and its slow ray parallel to the elongation direction of the sillimanite prisms, has been inserted.  The interference colour of the sillimanite has increased, by a value of 530 mµ (the retardation of the accessory plate), which causes the first order reds to appear as second order reds.  All regions of the slide have changed by the same amount, including the quartz and feldspar grains, but their various orientations have caused some to increase and some to decrease.  Isotropic regions (garnets and holes) and extinct grains have magenta colour (that due only to the 530 mµ retardation of the plate) and opaque minerals remain opaque.  The increase in the interference colours of the sillimanite grains indicates that their slow vibration directions are parallel to the slow vibration direction of the plate, and sillimanite has thus been shown to be length slow.  In C, prisms of tourmaline have interference colours of mostly first order orange to red.  Inserting the 530 mµ accessory plate causes the interference colours to decrease (D), and the tourmalines are mostly near black or first order grey.  This indicates that subtraction has occurred, and therefore tourmaline is length fast.