CHARACTER OP THE PRINCIPAL ZONE. 71 
PLEOCHROISM AND ABSORPTION. 
With the exception of the Ives colorimeter method for designating colors, 
the writer has had no opportunity to use other than the standard methods 
for the determination of these two properties.* At present the methods 
for investigating absorption phenomena in minerals are complicated and 
not in general use by petrologists. Such phenomena, however, are essen- 
tially quantitative in nature and in time will undoubtedly be included in the 
list of properties to be determined by exact methods. 
CLEAVAGE AND CRYSTAL HABIT. 
The methods for ascertaining these characteristics of minerals in the thin 
section are described at length in the standard text-books on petrology and 
need not be repeated here. 
CHARACTER OF THE PRINCIPAL ZONE. 
The determination of the position and character of the ellipsoidal axes 
ft and C in any mineral section is one of the most common problems in 
thin-section work and is satisfactorily accomplished by standard methods 
involving the use either of a sensitive tint plate cut from selenite or from 
quartz parallel or normal to the axis, or of a quartz wedge or a quarter 
undulation mica plate. The relative value of the ellipsoidal axes is ascer- 
tained by noting, on insertion of the wedge or plate, the rise or fall of the 
interference color in the crystal section under examination. Many minerals, 
however, are deeply colored and the natural color of the mineral is so in- 
tense that it veils seriously the interference color, so that it is often difficult 
to recognize the true succession of the interference colors. The same holds 
true for thick sections of strongly birefracting minerals. For these cases, 
in particular, the second standard method which was used by Fouque" at 
least 20 years ago) renders good service. This method is based on the 
direction of motion of the interference bands along the wedge-shaped 
edges of the mineral plate or grain, whether away from or toward its 
center (Fig. 43), on insertion of the quartz wedge. If like ellipsoidal 
axes in wedge and mineral plate coincide, the path difference between 
the emergent waves is increased on insertion of the wedge and the inter- 
ference bands appear to have moved from the center of the plate toward 
the margin ; if like ellipsoidal axes do not coincide, the reverse phenome- 
non is observed and the interference bands move from the margin of the 
plate or grain toward its center, as indicated in Fig. 43. 
THE COMBINATION WEDGE. 
In actual work with any one of the plates or wedges noted above, the 
observer contends with the disadvantage that the interference color of 
the mineral section in the slide rises or falls abruptly to some other inter- 
ference color on insertion of the plate or wedge. The thin edge of the quartz 
*H. Rosenbusch. Mikroskop. Physiogr. ist. ed., 105, 1873; P. Clan. Ann. de. Phys. N. F.. I, 331, 1877; 
Th. Liebisch. Phys. Krystallogr.. 520. 1891; Abbc-Zciss OcuUrspcctroscope in Rosenbusch- Wulfing (i). 
1. 349. 1904; J- Koenigsberger, Zeitsch. f. Instrumentcnkunde. 21, 129. 1901. 
