Page 211 - A Practical Introduction to Optical Mineralogy

.J (a) Direction of insertion NE to SW (denoted tf) '-/(c) Direction of insertion NE to SW (denoted -/ )
accessory plate: length slow length fast accessory plate: length slow length fast

0 0 1sogyre a 00
Y yellow 00

/ ~
biaxial +ve o'<- -ve Key +ve -ve
B blue
biaxial +ve -ve +ve +ve
~
00 B y 0 00 00
field ofviewD
~
( crosswi res
removed)
uniaxial +ve -ve isogyre y B +ve -ve y B B y
uniaxial +ve -ve -ve -ve

(b) Direction of insertion NW to SE (denoted"')
accessory plate: length slow length fast

00~ 00 (d) Direction of insertion NW to SE (denoted'>.) length fast

accessory plate: length slow
'.0
biaxial +ve -ve +ve -ve
0 'l..B ·o o:y biaxial +ve OAPO
\.
-ve
y B
B
y
O
uniaxial +ve -ve +ve - ve y "QB

Figure 4.20 Determination of optical sign of interference figures using either length slow or length Y B B Y
fast accessory plates, and with their direction of insertion either NE-SW or NW-SE. In the sets of
uniaxial +ve - ve -ve -ve
cartoons, numbers (a) and (d) are probably the most helpful. In (a) and (b), the uniaxial cross is placed
in the lower left-hand corner of the field of view, and the biaxial single optic axis isogyre is rotated until
it is concave towards the north-east. The cross and isogyre can, however, also be placed in the lower '+
10
right-hand corner of the field of view with the isogyre concave towards the north-west. This is shown in (t-)
(c) and (d).