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Click Here For Best Selection Of High Quality Polarizing Microscope

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We are now to discuss the reasons why a section in one position gives a polarization color when viewed between crossed nicols of the geological polarizing microscopes, and in another becomes extinguished. It has already been seen that first, if the light leaves the section vibrating as it entered (as it left the polarizer) it must suffer elimination in the analyzer; second, that a doubly refracting section has two rectangular vibration directions, which are the only possible directions of vibration in the section; and third, that extinction takes place at intervals of 90 degrees.

 

            If supposed, the mineral section is in such a position on the stage of the petrographic polarizing microscope that its two vibration directions coincide with the cross-wires, N.S. and E.W., the light emerging from the polarizer will travel through the section vibrating exclusively along the E.W. vibration direction in the section. And will emerge from it still vibrating E.W., that is, in such a way as to be totally reflected by the analyzer of the polarizing microscope. A similar result must be obtained if the section is turned through 90 degree, for then the other vibration direction has come to lay E.W., and again total reflection is effected by the analyzer. Such positions of the section under polarized light microscopes, then, must secure extinction.

 

            The reason for the production of polarization color when the vibration directions are not parallel to the cross-wires is not quite so simple. But it is not very difficult to obtain some idea as to the causes. It must be realized that the two vibration directions in the section are the only two possible for light penetrating the mineral, and also that while each vibration direction secures retardation of the light, the light that has vibrated in one of these directions has suffered a retardation, which is not equal to that suffered by the other. Suppose that these directions make angles of 45 degree with the cross-wires. The light emerging from the polarizer of the petrographic polarizing microscope is vibrating E.W. On entering the section it divides. Some vibrates N.E. is to S.W., the remainder N.W. is to S.E. The two rays travel up to the analyzer, and although there must be some resolution of the two into N.S. and E.W. vibrations, the remainder reaches the analyzer vibrating in other directions. The essential point is that this light has traversed the section vibrating in different directions, and consequently there is a relative retardation. On entering the analyzer of the polarizing microscope for geologists, the vibration directions are reduced to two only, namely E.W. and N.S. The E.W. vibrations are totally reflected from the film of balsam, and it only remains to consider the N.S. vibrations. Some portion of the light has traversed the section vibrating N.E. is to S.W., the remainder has traversed the section vibrating N.W. is to S.E. That is, there are two rays vibrating in the same plane with a difference of phase, and this difference is comparable with the wavelengths of light. Clearly then, the conditions are such as to produce interference, for it is evident that for at least one of the constituents of white light, the half wavelength must divide into the relative retardation so as to give some odd whole number. Disregarding other kinds of interference, this particular kind must result in the elimination of the color in question. The complementary color (the polarization color) penetrates the analyzer of the polarized microscope, and reaches the eye through the lenses of the microscope. The viewer will see that the polarization color depends entirely on the amount of the relative retardation effected between the two rays traversing the mineral section. This is quite unlike the case of the pleochroism of minerals in polarized light of the petrographic polarizing microscope. This pleochroism depends on the difference between the absorptive powers of the two vibration directions.

 

            There are several factors that must be taken into account in considering the difference between the two retardations. These are the thickness of the section, the crystallographic orientation of section, and the nature of the mineral. First, it is important to consider that the polarization color is dependent on the thickness of the section in the case of anisotropic minerals. This is beautifully exhibited or demonstrated when a wedge-shaped fragment is placed between crossed nicols. Crystalline gypsum is also suitable material for experiment. This mineral, which possesses a perfect clinopinacoidal cleavage, should be cleaved into a flake about one sixteenth of an inch in thickness, and from this a small rectangular measuring about 1 in. x ½ in. should be cut with a knife, care being taken that the edges of the rectangle are paralleled to the vibration directions. This can be done drawing an E.W. or N.S. line on the flake when it is in extinction between crossed nicols of polarizing microscope for geologists. The rectangle should be cemented with Canada balsam near the end of a glass slip, half an inch in width, and carefully ground down so as to make one end as thin as possible, while retaining the full thickness at the other. When such a wedge is mounted and placed between crossed nicols of the petrographic polarizing microscope, in N.E. is to S.W. or a N.W. is to S.E. position, it is seen to exhibit the succession of gorgeous color bands known as Newton’s scale. Every viewer should take an early opportunity of examining this scale, as no amount of description will serve to give an adequate idea as to the nature of the colors. It will be necessary to refer repeatedly to this scale, but for the present it may be regarded as a complete proof that the particular polarization color produced depends on the thickness of the doubly refracting mineral through which the light has passed, the greater thickness effecting a greater relative retardation. Secondly, the equal thickness of the same doubly refracting mineral produces different polarization colors when the sections have different crystallographic orientations, may be easily proved with almost any such mineral. Quartz supplies a very suitable case. If three sections are cut, say, parallel to the basal pinacoid, the rhombohedron, and prism, ground down to the ordinary thickness, and mounted, they will show quite different colors between crossed nicols of the geological polarizing microscopes. The basal section is isotropic while the other two are anisotropic. The rhombohedral section of the mineral is polarizing in gray color while the prismatic section is in yellow color. Barite is another common mineral giving good results. The macropinacoidal section polarizes in yellow while the brachypinacoidal section polarizes in bright pink or green. Third, the polarization color depends on the nature of the mineral and this may be demonstrated by mounting prismatic sections of Quartz and Calcite side by side. The Quartz polarizes in yellow while Calcite in pale gray.



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Friday, February 15th, 2008 at 5:09 am
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Characters of Minerals Between Crossed Nicols
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