992 The American Naturalist. [November, 
Determination of Optical Sign in Random Mineral Sec- 
tions.— Using the universal microscope stage Federow’ shows that it 
is possible and usually quite easy to determine the optical character of 
a mineral from random sections. In the case of uniaxial minerals the 
section is revolved between crossed nicols to extinction. It is then 
tilted first about one and then about the other axis of its ellipse of elas- 
ticity. The one of these corresponding to the ordinary ray is distin- 
guished by the resulting slight change in double refraction (due entirely 
to increase of thickness of the slide). Having determined this direc- 
tion (n,) it is only necessary to determine by use of the quartz wedge 
or mica plate whether this direction corresponds to the greater (posi- 
tive) or less (negative) elasticity. In the case of biaxial minerals a 
section is sought having the highest double refraction (nearest plane of 
optic axes). This is now tilted until it gives the lowest possible 
double refraction, when the light comes through it most nearly along 
an optic axis. If the angle which this direction makes with the axis 
of least elasticity (nearly in the plane of the section) is less than 45° 
(half the optical angle) the mineral is positive, otherwise negative- 
This latter method is only approximate, but is accurate enough for 
minerals having an acute optical angle of 75° or less, and these are the 
only ones in which determination of the optical sign is of much value 
for purposes of identification. 
Pseudochroism and Pseudodichroism.—The same author® 
furnishes an explanation of certain variations in color which are often 
observed in minerals having a lamellar structure when observed under 
the microscope. A bundle of white rays incident on any inclined 
plane separating two lamelle is in part totally reflected, the reflected 
portion being obviously made up of more rays from the violet than 
from the red end of the spectrum. Of the light which is transmitted 
the red rays are the less refracted, and hence take their direction nearer 
the axis of the microscope. Asa consequence the color observed near 
the centre of the field is due to the mixing of the red rays with the | 
darkness due to partial total reflection, and it is, therefore, brown. 
Nearer the margin of the field the more refrangible rays produce green. 
This effect is observed in ordinary (non polarized) light, and v. Fede- 
row proposes to call it pseudochroism. If the polarizer is used the 
amount of total reflection will evidently be greatest when the direction of 
vibration of the incident light is parallel to the surface of incidence, 
1 Ibidem, pp. 603-605. 
*Tscherm. min. u. petrog. Mitth. , XLV, heft 6. 
