182 
METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
In the first of these methods the horizontal position of the section is 
exactly that of the above method (Fig. 112). A\, having been previously 
located accurately, is brought to coincidence with ON, and the extinction 
angle of the specimen in the horizontal position is ascertained; and then, 
instead of being rotated about the horizontal axis V\ (the line OL in pro- 
jection), the section is rotated about V* (or ON in the projection) as an 
axis,* a given angle (apparent angle in air corresponding to angle in crystal) 
A i travels during the rotation of stage to A\ in the projection, the direction 
FIG. 1 1 1 . The general method of extinction curves shown in this figure is applicable 
to all sections in which one optic axis A i can be brought to coincidence with the axis 
of the microscope. After the determination of the exact position of A t by means of 
optical curves the specimen is rotated about H t until AI coincides with the plane NO 
normal to the axis V t of the universal stage. The extinction angle MOE of the specimen 
in the horizontal position is then determined; by construction EOA t is made equal to 
MOE; the specimen is then rotated about K t a convenient angle (apparent angle 
observed to be reduced to true angle in crystal), and the new extinction angle MOE' 
is ascertained. In the new position, the optic axis is contained in the plane OA ', angle 
E'OAi having been made equal to MOE'. The exact position of A t is then determined 
on the drawing on tracing-paper by noting the small circle of the underlying projection 
plat, whose arc A t A t ' intercepted between OA and OA t ' is equal to the angle of rotation. 
This determination can be checked by drawing the great circle CF, which marks the 
position which the plane OA t ' would assume were the specimen turned back to its 
original position. In practice the position of A , is determined for different angles of 
rotation about V l and the mean position of all determinations taken as the most probable 
and correct location of A t . 
of extinction wanders from OE to OE', and the plane OA j (containing At) 
wanders fromCMi toCMj'.the angle E'OAt being by construction =E'OAi. 
By recording the angle of rotation of the stage about ON ( Vt) required to 
bring the section to its new position, it is not difficult to find in the projec- 
tion that small circle, parallel toOL, whose arc AtAt (intercepted by the 
lines OAt and OAt) is equal to the above angle of rotation, and thus to 
locate At. To insure accuracy, this measurement should be repeated for 
several different angles of rotation and At determined in each case. As in 
The Mote effect can be produced by rotating the specimen 90 about Hi and then about Vi as an axis. 
