62 
MKTHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
the objective on the upper surface of the condenser and noting that the 
field does not shift appreciably in the different positions. A slight shift is 
of little consequence in its effect on results of measurement and can, there- 
fore, be neglected. Its correction is not an easy task and is fortunately 
rarely necessary. 
(2) Although many methods are available for the accurate crossing of the 
nicols, the following method is perhaps the simplest and most accurate : Re- 
move from the microscope all lenses ocular, objective, and condenser 
and point it directly at the sun, whose rays are parallel and so intense that 
a rotation of less than i ' of one of the nicols from the position of total extinc- 
tion is readily discernible. If the lower nicol be of the usual type, with 
oblique end surfaces, the rotatory effect of these faces on the plane of 
polarization of transmitted light-waves is such that it is advisable to close 
the diaframs of both the substage and the draw-tube so that only central 
FIG. 41. In this figure the point P of the sphere, corresponding to the direction CP 
within the crystal and located in this instance by the great circle A IP and the small 
circle DPK, becomes E in the stereographic projection plat and is there located at the 
inu-rsection of the great circle AEB, the stereographic projection at A IP, and by the 
small circle DEL, the stereographic projection of DPK. E is also the point of inter- 
section of the line OP with the horizontal diametral plane CGB. 
rays pass through the prism. The square-end type of prism, either Glan- 
Thompson or Ahrens, does not rotate the plane of polarization of trans- 
mitted light waves to the extent of the ordinary nicol and is therefore to 
be preferred in accurate work. For satisfactory work it is essential that 
the analyzer (Thompson prism with square ends) be mounted with its end 
surfaces normal to the axis of the microscope, otherwise the field is shifted 
on insertion of the analyzer. 
(3) For the adjustment of the cross-hairs of the ocular and the principal 
planes of the nicols, a mounted cleavage plate of some mineral showing 
parallel extinction, as anhydrite or anthophyllite or a crystallite of quartz 
with sharp prism edges, is observed under the microscope fitted with tin- 
objective and ocular but not with condenser. The microscope is pointed di- 
rectly at the sun and the position of total extinction of the plate determined 
readily to within i' of arc. The cross-hairs of the ocular are then adjusted 
to parallelism with the cleavage edge of the plate in its position of total 
extinction. For still finer adjustment the rotatory effect of tin- inu-rvcu- 
