in Rock-sections by maximum Birefringence. 261 



the vertical axis and in biaxial crystals in a direction parallel to 

 the plane of the axes of greatest, and least elasticities, that is to 

 say the plane of the optic axes. In these cases the difference 

 between the indices of the axes of greatest and of least elas- 

 ticity is the maximum birefringence, since it indicates the maxi- 

 mum eccentricity that can be obtained in any elliptical section 

 of the ellipsoid of elasticity ; for this, therefore, the strongest 

 interference colors are shown. We have prepared a table of 

 the values of maximum birefringence which is given on page 

 264. These values, which have been carefully collated from 

 the best and most recent determinations, are given in descend- 

 ing order and the list embraces the great majority of rock- 

 forming minerals. 



The recognition of sections of a mineral in a rock slide which 

 exhibit the maximum birefringence is somewhat inadequately 

 treated in most handbooks, it being generally assumed that the 

 student will form his own method from the optical data given. 

 It involves the determination of the " order " of the inter- 

 ference color displayed. It rarely happens that the bounding 

 walls of a mineral section in the slide are exactly perpendicu- 

 lar to the plane of the object and cover glasses; on the con- 

 trary they are apt to be inclined or in many cases have the 

 form of a thin wedge. From the thin edge of such a wedge, 

 especially by the aid of a high power and where the colors are 

 not interfered with by the overlapping of some other mineral, 

 as at the edge of the section, or near a crack or hole, or against 

 some isotropic mineral or one cut nearly normal to an optic 

 axis, it is often possible to count the successive colors in 

 ascending order, finally arriving at the top and noting the color 

 given by the full thickness of the section. This is essentially 

 the same process as moving up the thickening quartz wedge 

 and noting the colors it yields. If the ascending slope to the 

 top of the section is extremely steep the stronger colors of the 

 first order may be so crowded as to appear almost like a dark 

 line. If by any chance this process cannot be used, then the 

 section must be brought to extinction by the quartz wedge, the 

 section then removed and the color in the wedge noted and its 

 order ascertained by slowly withdrawing the wedge and noting 

 the successive orders of colors down to zero. 



The colors also vary with the thickness of the section. 

 Good sections as made to-day should not vary sensibly in thick- 

 ness in different portions, that is the two surfaces should be 

 parallel. They should also average about O03 mm or 0*001 inch 

 in thickness, or the maximum color exhibited by quartz should 

 not be above clear white or at most pale straw-yellow. 



With this thickness for the slide the majority of rock-mak- 

 ing minerals show maximum colors in the first and second 

 orders, colors easily remembered and determined. 



