78 METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
The importance of excluding all light except that from the mineral section 
or grain under examination was emphasized in the Introduction and a small 
device with adjustable aperture was described for the purpose (Plate i, 
Fig. i). This device was constructed primarily to replace the sets of cir- 
cular stops which were formerly furnished with the petrographic micro- 
scope, but which allowed the observer to stop off only a particular portion 
of the field; in consequence of their lack of adaptability they have been 
gradually discarded. The ordinary iris diafram in the draw-tube of the 
microscope is located too far distant from the eye to be of much service in 
the direct observation of interference figures with the unaided eye, because 
of the diffraction phenomena which arise when small apertures are used 
and which destroy the definition. If the objective be properly corrected, 
the stop in the image plane is effective and interference figures from grains 
o.oi to 0.02 mm. in diameter and of medium birefringence can readily be 
obtained.* 
PLATE PERPENDICULAR TO THE ACUTE BISECTRIX. 
For birefracting minerals in which 2E is less than 140 the methods for 
ascertaining the optical character ordinarily described in text-books are 
applicable and satisfactory. Both optic axes appear then in the field and 
the optical character can be ascertained either in convergent or in plane 
polarized light by the usual methods. If the optic axial angle is so large 
that the optic axes do not appear in the field of vision, the aperture of the 
objective can be increased by using homogeneous immersion, and if this be 
not sufficient an approximate idea of the value of 2E can be obtained by 
Michel-Levy's methodf of noting the angle of rotation of the stage from 
the position where the zero isogyres of the interference figure form a cross 
to that at which they are tangent to a given circle (usually margin of the 
field of the microscope). From this angle ?E can be determined and from 
it in turn, the correct optic axial angle (2 V), if the medium refractive index of 
the mineral plate be known. This method, however, is not in general use 
because of the inaccuracy of the results obtained and of the calculations 
involved, the microscopist preferring usually to find another section on 
which the optical character can be ascertained directly and with greater 
accuracy. 
PLATE PERPENDICULAR TO AN OPTIC AXIS. 
The interference figure obtained from this plate consists ordinarily of a 
black achromatic bar which, on moving the stage, rotates in a direction 
opposite to that of the stage. This bar is in general a straight line only 
when it is parallel to the planes of polarization of the nicols. In the inter- 
mediate positions it is more or less convex, depending on the angle between 
the optic axes. If 2 V, however, is equal to 90, the curve is a straight line 
In weakly birefracting trains (unUxial or biaxial) the field between crossed nicols in convergent polarized 
light U to dimly lighted that, on the insertion of the sensitive tint plate, it is fluoded with light and the 
delicate differences iu tint caused l>y the crystal plate are often difficult to recognize and may render the 
determination uncertain. In such instance!!, the sensitive tint plate below the condenser .'. I'lati i. I ; IK ',< 
should IM: rotated until it axes are almost parallel with the principal nieol planes, in which position (he 
intensity of illumination from the sensitive tint plate itself is very slight, while the path differences remain 
unchanged and the differently colored quadrants or areas stand out in sharp coin 
t Michel Levy. I.es Mineraux des Roche*. 94-95, 1888. Compare also F. E. Wright, Amer. Jour. Sci. 
(4). 20. 287. 1905; H. Tertsch. Tscherm. Min. Petr. Mitteil. 27, 389-594, 1908. 
