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 appearauce 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 dimetrice 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-ecting the acute axial angle, is view ed 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 Nicol prism, or tourmaline plate, an unsymmetrical 
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GLAUBER SALT. PHLOGOPITH, ANTWERP, N. Y. 
black crosx is also seen, as in fig. 4; if it makes an angle of 45° 
with this, {wo curved black bars are observed, as in fig. 5. In 
either case the colors are 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 bet ween the axes is 
50° to 70°, and, if the tourmaline tongs are employ ed, thetwo 
