When confusion arises, the exact order of the color can be determined by a quartz wedge, as given on p. 35. Furthermore, a ¼ undulation mica plate serves to quickly distinguish between the 1° order white and the practically colorless, high order tint of calcite, titanite, etc.; as after the insertion of the test-plate the 1° order white suffers a marked change in color, while the very high order (practically white) tint shows no appreciable change. In the determination of these interference colors care must be given to considerations of orientation and thickness. The section must give the maximum interference color of all the obtainable sections of the mineral in the rock section. Such sections will be parallel to the _ć_ axis in the uniaxial minerals and to the axial plane in the biaxial minerals; and will, therefore, in convergent light never show the emergence of an optic axis or bisectrix. Crystal “form,” cleavage, pleochroism, etc., may at times aid in the selection of these sections. The thickness of the section must also be considered, and it is well in all cases to pick out some known mineral in the section, as quartz, and note its maximum interference color. Knowing how this varies from the color given by the scheme for a section 0.03 mm.[159] in thickness, due allowance can be made for a like variation in the colors given by the other minerals. In the case of minerals with strong absorption the interference colors may not be noticeable on account of the absorption of parts of the light. Under the subhead of pleochroism, the vibration direction of the ray of a definite color is given and not the direction of transmission of that ray. The interference figures in convergent light increase in clearness and distinctness with the strength of the double refraction. In uniaxial crystals sections at right angles to the optic axis, _i. e._, sections which remain dark during a revolution between crossed nicols, show the best interference figures. In biaxial crystals the most characteristic interference figures are shown by sections at right angles to the acute bisectrix. Crystal sections which are too small do not give very satisfactory interference figures with convergent light.[160] Any scheme, however designed, makes a more or less arbitrary classification of the minerals, and when in doubt it is always safer to look for the mineral on both sides of the scheme line. The rarer minerals are not included in this scheme, so when the determination of a mineral is uncertain or not positive recourse should be had to more elaborate tables. INDEX. MINERAL NAMES IN HEAVY-FACED TYPE. Abbreviations, ix Abnormal interference colors, 30 Absorption, 26 directions, 26 tints, 17 ▄Acmite▄, 83 ▄Actinolite▄, 84 Acute bisectrix, 5 ▄Ægirine▄, 83 ▄Ægirine-augite▄, 84 Aggregate structure, 38 ▄Albite▄, 103 ▄Allanite▄, 96 Allotriomorphic, 15