134 
BULLETIN OF THE LABORATORIES 
plane corresponding to i-i = i-K and coincides with a symmetiy plane, hence coal- 
escence in this way would not produce twins. A second type is the pericline type 
in which the twinning plane is at right angles to the zone O, i-I. 
Pleochroism. 
Double refracting minerals have the property of affording different colors when 
looked through in different directions corresponding to the axes of elasticity. In 
optically uni-axial crystals there are two such directions {i. e. the minerals are di- 
chroic) and the color afforded by looking through in the direction parallel to the 
vertical axis is called the basis color, that appearing when looked through at right 
angles to this the axial color. A section of a uniaxial crystal will show no 
change of color when rotated above the polarizer (analyzer being removed) if the 
section is parallel to the vertical axis. 
In tetragonal and hexagonal minerals, therefore, the directions where greatest 
change of color occur coincide with the two axes of elasticity, in the orthorhombic 
system they coincide with the three crystallographic axes as wel', but in the mono- 
clinic and tnclinic this coincidence seems not to occur. 
Use of convergent Polarized Light. 
The occular is removed and a condensing lense is placed above the polarizer, 
between it and the object. Interference figures of a nature varying with the 
character of the mineral now appear. In regular and amorphous minerals no 
such figures are produced. The same is true of sections parallel to the vertical 
axis of the hexagonal and tetragonal minerals. In the transverse (isotropous) 
sections of tetragonal and hexagonal minerals an invariable dark interference cross 
lies in the centre of the field. If the sections is oblique, the cross falls at one 
side of the centre but is not otherwise altered. A rotation of thesta^e causes the 
cross to apparently revolve in the same direction. If the section is so oblique 
as to fall outside the field a rotation will bring first one limb and then the other 
into view. 
A section of biaxial crystals taken perpendicularly to the bisectrices and 
placed so that the optical plane coincides with the principal section of the Nicols 
shows two closed curves enclosing the axial points. These curves are bordered 
by other curves and crossed by a dark cross. The smaller limb of the cross 
passes through the axial points and indicates the position of the optical axis 
plane. The broader limb of the cross is at right angles to it. When the 
stage is revolved the cross does not remain invariable but is altered to form two 
hyperbolas which move about the aixial points and again form a cross after a 
revolution of go°. 
The above account, condensed, in the main, from Hussak, will serve to in- 
troduce the tables, while the student may be referred for more full explanations to 
the works of Rosenbusch, Zirkle, Fouque^ and Levy, Cohen and especially 
Hussak’s “ Anleitung zum bestimmen der gesteinbildenden Mineralien,” 
D. Recapitulation of civiracters of the various crystal systetns. 
I. Amorphous and regular minerals are distinguished from all others by re- 
maining dark in all positions between crossed Nicols, while the later may be 
