6 CONTENTS. 
CHAPTER IV "5 
Extinction angles 115 
Mathematical formulae for the intensity of light 115 
Methcxls 132 
Parallel polarized light 132 
Sensitive-tint plate 133 
Bravais-Stdber plate 1 34 
The Koenigsberger plate 134 
The double combination wedge 134 
Caldenm calcite wedge 135 
Quarter-undulation plate of H. Traube 135 
Twinned selenite plate 136 
Artificially twinned quartz plate 136 
Bi-nicol ocular 137 
Bertrand plate 13? 
The quartz half-shade plate of S. Nakamura 139 
Bi-quartz wedge-plate 139 
Methods involving rotation of the upper nicol 141 
Convergent polarized light 141 
The relative sensitiveness of the different methods 142 
Experimental tests 145 
CHAPTER V 147 
Optic axial angle 147 
Convergent polarized light 147 
The Mallard constant 148 
The Becke drawing-table method 152 
Methods with the double-screw micrometer ocular; also with the coordinate 
micrometer ocular 155 
Observations in convergent polarized light 162 
Muscovite 163 
Anhydrite 1 64 
The condenser apertometer 172 
Summary statement of course of procedure in measuring the optic axial 
angle of a mineral plate in convergent polarized light 172 
Michel-Levy Method 1 73 
Methods with the universal stage 1 73 
Parallel polarized light 175 
The determination of the crystal system of a given mineral by means of the 
universal stage 177 
The accurate determination of the position of an optic axis in the field of 
vision 178 
Section nearly perpendicular to the optic normal 186 
Section nearly normal to the obtuse bisectrix 187 
Probability of encountering sections suitable for optic axial angle measure- 
ments 189 
Methods based on the relative birefringence of known sections 190 
Extinction angles of plates in zones whose axes lie in the plane of the optic 
binormals 191 
irement of the optic axial angle on the total refractometer 193 
The relative- accuracy of the different methods 194 
Measurements with the axial-angle apparatus 194 
Measurements with the Becke drawing-table 194 
Measurements with the double-screw micrometer ocular 196 
Measurements with the universal stage 197 
Recapitulation. 199 
