crossed nicols

Pyrargyrite (blue under plane light) has strong red internal reflection under crossed nicols as shown here.

Photomicrographic under crossed nicols of the entire annular shear zone at different finite strains.

Granular stibnite, crossed nicols, showing deformation twinning and intense anisotropism.

This leaves the insoluble components available for observation in a dry state by reflected light or transmitted light with crossed nicols.

Between crossed nicols the fibers extinguish parallel to their length, and they do not show an optic figure in convergent polarized light.

Margin of a marcasite stalactite under crossed nicols showing anisotropism of marcasite blades and structure at margin of stalactite.

If they belong to uniaxial crystals they show a dark cross or convergent light between crossed nicols, the bars of which remain parallel to the wires in the field of the eyepiece.

One minor criticism we would make is that most of the feldspars, photographed at or near extinction with crossed nicols show a rather brownish colour, somewhat similar to that shown by moderate dispersion of the refractive indices.

If ordinary light and not polarized light is desired, both prisms may be withdrawn from the axis of the instrument; if the polarizer only is inserted the light transmitted is plane polarized; with both prisms in position the slide is viewed in cross-polarized light, also known as "crossed nicols."

To facilitate measurement of extinction angles various kinds of eyepieces have been devised, some having a stereoscopic calcite plate, others with two or four plates of quartz cemented together; these are often found to give more exact results than are obtained by observing merely the position in which the mineral section is most completely dark between crossed nicols.