68 PHYSICAL PROPERTIES OF MINERALS. 



faces, and a similar tourmaline plate in front of the opposite 

 face, the colored rings will be seen on looking through ; and by 

 revolving one of the tourmaline plates a change will be observed 

 at each 90° of revolution, in the colors of the rings, and in the 

 variations in appearance of the cross from black to white, and 

 the reverse. The fact in any case that the rings of color are 

 perfect circles, and the black cross a symmetrical one, is proof 

 that the crystal is either of the dimetric or hexagonal system. 

 But sometimes very exact observation is necessary to deter- 

 mine the truth. 



3. Crystals Biaxial in Polarization. — Biaxial crystals are 

 those having two optic axes, and the angle between them is 

 called the axial angle. 



When a section of such a crystal, at right angles to the line 

 bi>eeting the acute axial angle, is viewed in converging polar- 

 ized light, the two axes are seen with a series of elliptical col- 

 ored rings surrounding each. If the section is so placed that 

 the line joining the axes coincides with the vibration-piane of 

 either ISicol prism, or tourmaline plate, an unsymmetrical 



AS^K 





PHLOGOPITE, ANTWERP, X. Y. 



black cros< is also seen, as in fig. 4; if it makes an angle of 45° 

 with this, two curved black bars are observed, as in fig. 5. In 

 either case the colors nre reversed, and the black changed to 

 white as one of the Nicols is revolved. Fig. 6 shows the 

 axial figure for phlogopite (in the second position mentioned 

 above) where the axial angle is very small. The rings are less 

 numerous and farther apart the thinner the section that is 

 employed in making the observations. 



In muscovite (common mica) the angle between the axes is 

 50° to 70°, and, if the tourmaline tongs" are employed, the two 



