Alumina with Silica, Lime and Magnesia. 331 



able error of about ±"002. The Becke line method can also 

 often be used to advantage with refractive liquids. The 

 refractive index of the liquids or mixtures of liquids used is 

 determined on the total refracto meter. 



(2) Birefringence. — In this measurement the thickness of 

 the plate is measured with the fine adjustment screw of the 

 microscope by focusing a high power objective first on the upper 

 surface of the plate or grain and then on the lower surface as 

 it appears through the plate or grain itself. The apparent 

 thickness thus obtained is then reduced to the true thickness 

 by multiplying by the refractive index of the substance 

 measured. The interference color is determined either with 

 the Babinet compensator, the calibrated quartz wedge or the 

 Michel-Levy interference-color chart. This is only an approx- 

 imate method, and the results may be in error 10 per cent or 

 more, but usually the percentage error is less and the numerical 

 error is confined to the third decimal place. 



(3) Optic axial angles are measured most readily in the 

 powder section by means of the double screw micrometer 

 ocular.* On favorable sections ( - 025 mm and over in diameter), 

 the probable error of such measurements is about ± 1° in case 

 both optic axes appear in the field of vision, and ± 3° in case 

 only one optic axis is seen. For such measurements the grains 

 should be immersed in a liquid of the refractive index /3 to 

 eliminate errors caused by refraction on the uneven surfaces 

 of the grains. In weakly birefracting substances and inter- 

 rupted sections the axial bars are less sharply defined and the 

 axial angle values obtained thereon are correspondingly less 

 accurate. 



(4) Extinction angle. — By use of the bi-quartz wedge platef 

 the position of total extinction can be determined on a single 

 trial within db 10' on favorable sections. The extinction angle 

 itself is the angle between a given crystallographic direction 

 and a particular optical direction on a particular crystal face, 

 and the accuracy with which it can be determined depends in 

 part on the quality of the crystallographic development of the 

 crystallite itself. Under favorable conditions of crystallo- 

 graphic development, extinction angles can be determined 

 within 10' and less, dependent on the number of readings taken, 

 on crystals measuring only *02-*03 mtn in length. 



(5) Color, pleochroism and absorption can usually be deter- 

 mined on grains measuring •02 mm and over, and in certain 

 instances on still smaller particles. 



(6) Other properties, such as dispersion of the optic axes 

 and bisectrices, and the general correlation of optic and crystal- 

 lographic properties, can occasionally be accomplished on 



* This Journal (4), xxiv, 317-369, 1907. 

 f Ibid. (4), xxvi, 349-390, 1908. 



