96 METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
Under the same conditions, the accuracy of the Becke line method and 
of the oblique illumination method is about the same when applied to 
mineral grains immersed in refractive liquids. With monochromatic 
illumination (strong sodium light) the refractive index of an isotropic clear 
mineral grain can be determined with an accuracy of =to.ooi, even though 
the diameter of the grains is only o.oi to 0.02 mm. In case ordinary day- 
light illumination be used the accuracy for about 550 nn is less, about 
=*=o.oo2 to 0.003 on favorable sections. 
For birefracting mineral grains or plates the different refractive indices 
(7' and a') of the plate are ascertained by using plane polarized light and 
allowing the vibrations of the transmitted light to take place along the 
principal ellipsoidal axes of the section. The principal refractive indices 
of the mineral are determined by using grains normal either to a bisectrix 
or to an optic axis and determining the value of the different transmitted 
waves.* 
In general it may be stated that the method of oblique illumination and 
the Becke line method are universally applicable, but that in certain in- 
stances the one method is better suited to meet the conditions than the 
second, and vice versa. As a rule the method of oblique illumination has 
certain advantages, since it requires only that the field be shaded by placing 
the finger in front of the polarizer or reflector and is used with low-power 
objectives so that the relative refractive indices of the whole aggregate of 
sections in the field can be seen at a glance, while in the Becke line method 
a higher power objective is advisable, the substage diafram must be closed 
or the condenser lowered, and practically only one mineral grain can be 
tested at a time. The oblique illumination method throws at once the 
elements of the field into relief, and the contrast between the higher and 
lower refracting grains is sharply brought out. This last feature of the 
method of oblique illumination is especially useful in the determination of 
orthoclase in a thin section when it appears only in minute grains and is 
surrounded by plagioclases. The refractive indices of orthoclase are con- 
siderably lower than those of the plagioclases or of Canada balsam and 
its sections appear, therefore, as small pits in the general background of 
the slide and are readily recognized; whereas, if the Becke line method 
were used, the edges chosen for test might not include an orthoclase edge and 
the orthoclase might be overlooked altogether. 
In actual microscopic work, both with thin sections and with powder 
sections mounted in liquids, the two methods are used indiscriminately and 
often both are applied, the one serving as a check on the results obtained 
by the other. 
Objection might be made to the application of these methods, all of which 
involve oblique illumination, to birefracting minerals in which the refractive 
index varies with the line of propagation of the transmitted wave. For a 
general section this objection might have force and the results obtained 
be correspondingly inaccurate, but for the oriented grains, normal either 
to one of the ellipsoidal axes or to an optic axis, on which proper de- 
Por the determination of the relative refractive indices of two adjacent mineral plates in the thin section 
J. W. Evans (Centralblatt far Miner., 1910, 188) ha* recently recommended that the section be examined 
under parallel nicols with the planes of vibration of the adjacent crystal plates equidistant from the principal 
nicol planes. 
