MICROSCOPICAL PETROGRAPHY 403 
be detected.*. If the sensitive-tint plate be inserted in the diagonal 
position, it illuminates the field so strongly that the faint color 
differences from the mineral grain are veiled and often completely 
lost to view, especially if the grain be minute. The same method 
is applied with equal success to the determination of the optical 
character of very weakly birefracting minerals in convergent- 
polarized light. 
The optical character can be determined on all sections in which 
at least one optic axis appears in the field by noting that the con- 
vexity of the axial bar (zero isogyre) passing through the optic 
axis is always directed toward the acute bisectrix.? On sections, 
perpendicular to the optic normal the acute bisectrix can be located 
by noting that the faint achromatic hyperbolas of the interference 
figure pass out of the field in the direction of the acute bisectrix3 
on rotating the stage. In case the plate is so thick that the inter- 
ference colors are bright, the position of the acute bisectrix can 
also be ascertained by noting that the interference color is rela- 
tively lower in the quadrants containing the acute bisectrix than 
in the adjacent quadrants. In thin rock sections the mineral 
plates of weak to medium birefringence do not often exhibit 
interference colors much above pale yellow of the first order and 
for such plates the second method is inadequate and the first 
should be used. The relative value of the acute bisectrix whether 
tSee F. E. Wright, ‘“‘The Methods of Petro. Microsc. Research,’’ Carnegie Inst. 
Washington, Pub. 158, 73, 1911. In the writer’s microscope the sensitive tint plate 
is mounted in a rotating collar beneath the substage condenser and can be turned 
quickly from one quadrant to another, thus greatly facilitating its use in rapid 
routine work. 
2 The following concise statement of the rule for finding the sign of reaction of a 
section has recently been given by F. Rinne (7.M.P.M., XXX, 321-23, 1911): A 
central uniaxial interference figure divides the field into four quadrants. Let the 
NE and SW quadrants be considered positive as usual and the adjacent quadrants 
negative. In biaxial minerals the areas NE and SW of the achromatic brushes 
(zero isogyres) taken with respect to the acute bisectrix may in like manner be con- 
sidered positive. If now the sensitive tint plate be inserted so that the major axis 
7 of its optic ellipse points NE, SW, then the mineral is optically positive when the posi- 
tive quadrants or areas of the interference figure are colored blue and negative when the 
negative areas are colored blue. 
3F. E. Wright, Amer. Jour. Sci. (4), XVII, 387, 1904. 
4F. Becke, 7.M.P.M., XVI, 181, 1897. 
