This page collects images of biaxial conoscopy as a function of the thickness of
the crystal plate.
Video of the rotation of a
muscovite conoscopic image perpendicular to the acute bisectrix.
Same muscovite but double thickness.
Higher order isochromes are visible.
A single sheet of muscovite with a Benford plane. As usual, the isogyres
are canceled and the rotation of the poles of the optic axes are visible.
Images below were obtained with muscovite crystals. The
thickness of the plates increases from the top of the page to the bottom.
Figure 1a. Very thin plate with optic
plane horizontal. Only the isogyres can be seen. No isochrome can be seen
in the field of view due to the low birefringence.
Figure 1 b. Rotation of the microscope
stage at 45°. The isogyres are now hyperboles.
Figure 1c. Same orientation as figure 1a
but with a Benford plate inserted. Isogyres are removed, only the poles of
the optic axes are visibles.
Figure 2a. Thicker plate than in figure
1. Optic plane horizontal. Thinner isogyres close to the optic axes. The
first isochromes visible are part of a lemniscate.
Figure 2b. Hyperbolic isogyres at 45°
orientation. The isochromes simply rotate as the crystal.
Figure 2c. Same as 2a with Benford plate.
Optic axes clearly visible.
Figure 3a. O° orientation. Increased
thickness. The vertical part of the isogyres becomes wider. First order
isochrome is well developed, second order appears at the top and bottom of
Figure 3b. Orientation 45° with Benford
plate inserted. The first order lemniscate is nearly complete.
Figure 3c. Monochromatic light (467 nm).
45° with Benford plate. This figure and the following one show the
influence of the wavelength of the light on the shape of the lemniscate
moving around the poles of the optic axes. The second order extinction
becomes visible at this wavelength near the edges of the figure.
Figure 3d. Monochromatic light (628
nm). 45° with Benford plate. At this wavelength, lemniscate is wider.
Second order is not visible in the field.
Figure 4a. O° orientation. Increased
thickness. Monochromatic light 467 nm. Increased number of extinction
curves. Closed curves around the optic axes appear.
Figure 4b. Orientation at 45°.
Figure 4c . O° orientation with Benford
plate. If the stage is rotated with the Benford plate installed, the
figure simply rotates with no changes.
Figure 4d. Same image as figure 4a with
wavelength equal to 628 nm.
Figure 4e.Same image as figure 4b with
wavelength equal to 628 nm.
Figure 4f. Same image as figure 4c with
wavelength equal to 628 nm. Only one closed curve is present at this
wavelength around optic axes poles.
Figure 5a. The highest thickness of the
page. Vertical part of the isogyres is very wide. High birefringence
Figure 5b. 45° orientation with Benford
plate. The shape of lemniscates is clearly demonstrated.
Figure 5c. 0° orientation. Wavelength =
Figure 5d. 45° orientation. Good view of
the hyperbolic isogyres.
Figure 5e. 45° orientation with Benford
plate. Very sharp poles of the optic axes. Lemniscates of high orders are
Figure 5f. 45° orientation with Benford
plate. Fewer isochromes than the blue image.
Two superposed sheets of muscovite rotated by 90° from each other. This
gives this amazing conoscopic figure from two indices ellipsoid in quadrature.