136 
BULLETIN OF THE LABORATORIES 
rhombic piece is obtained by using the conchoidal fracture parallel the ortho- 
pinacoid and the satiny cleavage parallel to the -)- hemipyramid. The obtuse 
angle =113° 46^ and the acute = 65 ° 14''. In as much as the optical axes in 
gypsum lie in the clinodiagonal terminal plane, the surfaces of the plate may be 
taken as the plane of the optical axes. The conchoidal fracture corresponds with 
the main axis, and the optical axes above and anteriorly form angles of 23° and 
83°. These are represented. By bisecting the acute and obtuse angles we may 
secure the bisectrix (axis of least elasticity) and the optical normal (axis of 
greatest elasticity) respectively. The bisectrix iorms an angle with the longer 
diagonal of the plate of nearly exactly 20°. Every ray entering the plate per- 
pendicularly is polarized, forming one ray vibrating in the plane of the bisectrix 
and another in the plane of the optical normal. In a drawing of the exact shape 
of the plate these lines are drawn and, in addition, the whole area checked into 
squares by lines parallel to the bisectrix and the normal (on both sides.) The 
plate is laid upon the drawing and both are bisected parallel the main axis and 
one piece is revolved 180° and united by its other edge with the second. The 
result is an artifical twin of gypsum with the optical determinants all indicated in 
each (Rosenbusch.) The same method may be applied to illvistrate other twins. 
6 A. Orthoclase twins of Carlsbad type. 
B “ Banover type. 
7. Polysynthetic twins of calcite in marble. 
8. Section of augite parallel to i—i, showing zonary structure. (Hussak.) 
9. Micro- crystals of gypsum (twinned.) 
10. Inclusions showing Auction structure. 
II Hornblende crystal with opaque altered margin. (Hussak.) 
12. Diagrams of interference Agures in convergent polarized light. 
(Adapted from Fouque^.) 
The polarizer and conden.s.or are used, but the occular is removed and the 
Nicols are crossed. In regular and amorphic minerals no Agure is seen. In 
uniaxial crystals in isotropous sections there appears a Axed axial cross with more 
or fewer concentric colored rings. These latter vary with the thickness of the 
section. If the section is not exactly at right angles to the main axis the inter- 
section lies out of the centre and revolves with the motion of the section. If 
the inclination is still greater the intersection falls beyond the held and aTevolu- 
tion of 90° brings Arst one and then the other limb of the cross into view, 
I A= a section slightly oblique, I B is the same section revolved 45°, and I C is 
revolved °90. Biaxial crystals, if taken perpendicular to bisectrix or normal, 
and if so placed that the opical axis plane coincides with the principal section of 
the Nicols, show an interference Agure consisting of two separate systems of curves 
whose foci correspond with the two axes. These curves are surrounded each 
by a lemniscus and a black cross appears with a narrow arm passing through the 
foci and a broader band between them (II A.) The slender band represents the 
position of the optical axis plane. When the section is revolved the cross alters 
its form and at 45° becomes an hyperbola passing about each focus while at 90° the 
cross reappears but in a position transverse to that formerly occupied. 
