Van der Kolk's method is better for irregular fragments and is based upon the behavior of the fragment in obliquely incident light. With proper adjustments approximately lens-shaped fragments become more strongly illuminated on one side or the other according as their in dices of refraction are greater or less than that of the surrounding liquid.
Accurate determinations of the principal in dices of refraction are often preferably made by the "prism method" involving the measure ment of the "angle of least deviation" and of the prismatic angle. Up to a value of about 1.8 they are very conveniently made by the use of ref ractometers based upon the principle of total reflection, in which the indices of refrac tion can be rapidly determined upon polished or natural surfaces from a millimeter in diameter up.
In a much used type the surface of contact is the diametral plane of a glass hemisphere of very high index of refraction, the crystal sur face is placed on this with a drop of some liquid also of high index. The diffused light from one side is in part totally reflected and the °limit line" found by a telescope. The index is calculated from the angle at which the telescope obtains the adjustment Determining Birefringence.— The birefrin gence or strength of double refraction of any doubly refracting crystal is the difference be tween its maximum and minimum indices of refraction. If these indices can be determined to the third decimal their difference may be taken as the birefringence.
In practice the birefringence is usually de termined from the retardation A and conse quent interference color and the thickness of the section, by the relation A=t(ni--n).
The retardation A can be measured with great accuracy by compensators. The thick ness determination is less accurate.
The function of the compensator, which in its simpler form is a wedge of doubly refract ing crystal, is to lower the value of A by known amounts until Determining Extinction Angles.— The angle between an extinction direction and some rec ognizable crystalline direction, cleavage, crack, edge, twin plane, etc., is called an extinction angle of the section.
The best general method is to carefully de termine the 'position of the crystalline direction, then rotate the stage clockwise until the field is dark at some reading a. Continue the rotation until the field is light, then turn back counter clockwise to some reading a' where the field is again dark. The reading halfway between is near maximum darkness.
Test plates are also used which between crossed nicols develop some sensitive interfer ence color. This color is not changed when the section being tested is in its extinction position, but is notably changed for any other position.
Determining Pleochroism —Pleochroism may be determined with the polarizing microscope as follows: The extinction directions are found and then the upper nicol is pushed out and the colors for the two extinction positions noted and com pared. These positions give the maximum dif ference in color for the direction of transmis sion. The pleochroism may appear as a change in color or a change in the shade of the same color.
It is sometimes more satisfactory to observe the "twin" colors side by side by use of a di chroscope which in its essentials is a cylindrical casing with a rectangular hole at one end and a lens at the other and between a rhomb of calcite of such a length that the two images of the hole are just in contact.
The section is held close to the rectangular hole and the instrument rotated until the two images are alike in color. Midway between two positions the colors differ most.
Determining Uniaxia! or Biaxial by Inter ference Figures.— By introducing a convergent lens below the stage of the microscope polar ized rays are sent through the section in a great number of directions and a simultaneous pic ture is obtained of the interference phenomena due to the varying structure in all these direc tions. Usually a high power objective is needed and the so-called interference figure may be seen by removing the eye-piece and looking down the tube or by retaining the eye-piece and using suitable extra lenses.
No interference figure is obtained from any section of an isotropic (isometric) crystal. The field remains dark.