l8o ELEMENTARY CHEMICAL MICROSCOPY 



Since in feebly polarizing crystals the exact point of extinction 

 is not easily determined, a measurement of the angle is difficult 

 and annoying unless a selenite test plate is employed (see page 

 175). When employing a selenite proceed as follows: Place 

 the test plate, red of the first order, so that the plane of its 

 direction of vibration bisects the opposite quadrants of the cross- 

 hairs of the ocular. With the nicols crossed bring a typical thin 

 crystal so that its long edge (or its c-axis) lies parallel to a cross- 

 hair. A red field is seen with the crystal of some contrasting 

 color. Read the graduated stage circle. Now slowly rotate 

 the stage until the crystal acquires exactly the same color as the 

 field; the plane of vibration of the selenite and that of the 

 crystal are now coincident. Read the stage again. The read- 

 ing will give an extinction angle. Next ascertain whether it is 

 the smaller of the two possible angles for this position of the 

 crystal. Make similar measurements upon a number of other 

 crystals. 



Never depend upon observations made upon a single individ- 

 ual. Check the readings by again making the crystal parallel 

 to the cross-hairs and turning the polarizer or analyzer until 

 the colors of field and crystal are identical; read the gradua- 

 tions on the nicol mounting; the angles observed should be 

 identical. 



CHARACTERISTICS OF THE SIX CRYSTAL SYSTEMS. 

 SUMMARY. 



The chief characteristic features exhibited by individuals of 

 the six different crystal systems which will prove of assistance 

 in microchemical analysis may be summarized as follows: 



ISOMETRIC SYSTEM (Cubic System). 



The three crystallographic axes are all at right angles. Each axis is one of four- 

 fold symmetry. All axes are of like value, hence any axis may be made the c-axis. 



Cleavage usually parallel to the faces of the crystal and symmetrical with refer- 

 ence to the crystallographic axes. 



Optically isotropic, hence there is no change between crossed nicols. No inter- 

 ference figures. 



A single refractive index, independent of direction. 



