132 METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
polarized light and the general deductions from the latter may be considered 
to apply to the phenomena in convergent polarized light. The methods 
involving convergent polarized light, however, have several important de- 
fects which render their general application cumbersome and unpractical. 
In the foregoing pages, the intensity formulae for light transmitted by 
crystalline plates under different conditions have been developed and the 
attempt has been made to treat the subject in such a way that the results 
attained shall be directly applicable to the practical methods for determining 
extinction angles under the microscope. 
METHODS. 
Extinction angles can be measured either in plane-polarized light or in 
convergent polarized light; in plane polarized light the positions of total ex- 
tinction are determined either by observing the relative intensities of mono- 
chromatic light under special conditions or by means of the interference 
colors resulting from the use of white light; in convergent polarized light 
they are ascertained by noting the symmetry relations of an interference 
figure obtained from a specially prepared plate. In all measurements of 
extinction angles it is imperative that careful attention be given to the 
source of light, especially if monochromatic light be used. The source 
should be as intense and steady and uniform as possible in order that the 
variation in the source of light itself be not mistaken for actual differences 
in the microscopic field. The rays of light incident on the preparation 
should, moreover, be as nearly parallel as it is possible to obtain them. To 
meet these requirements satisfactorily requires both time and patience, but 
in order to attain the best results they can not be overlooked. 
Assuming the microscope to be in perfect adjustment and the source of 
light satisfactory, any one of the following direct methods may be used for 
measuring the extinction angle of a particular crystal plate. 
PARALLEL POLARIZED LIGHT. 
(i) The ordinary method, which consists in turning the crystal plate 
under crossed nicols until the position of maximum darkness is attained, 
is in general use and is equally well adapted for white light and for mono- 
chromatic light. With it any degree of accuracy can be attained provided 
a sufficient number of measurements be taken to reduce the probable error. 
In applying this method it is customary to note, not only the positions of 
maximum darkness attained by the crystal when rotated clockwise from a 
position of bright illumination, but also when rotated counter-clockwise 
from such a position. This was the method used by Max Schuster in his 
measurements of the extinction angles of plagioclase feldspars. He deter- 
mined for each cleavage flake the position of total extinction eighty times 
for clockwise rotations of the plate and eighty times for counter-clockwise 
rotations, and averaged the two readings. His work in this line remains 
unsurpassed, even to the present time. 
To increase the accuracy of each determination on a crystal plate under 
crossed nicols, different schemes have been devised, all of which depend on 
the disturbing influence of the plate on inserted plates or wedges of bire- 
fracting substances. Each of the inserted plates or wedges is constructed 
