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Click Here For Best Selection Of High Quality Polarizing Microscope

Click Here For Best Selection Of High Quality Polarizing Microscope

Up to the present wedge were used merely to demonstrate the effects of vibration in thickness of an anisotropic section of a mineral. Now, we have to consider some other uses to which wedge may be put.

 

            It has been suggested that, by practice, the beginner mineralogist may acquire the ability to recognize the particular order in which any mineral fragment is polarizing, merely by inspection. But the wedge may be used to place the matter beyond doubt. In order to demonstrate the method, a fairly large fragment of a doubly refracting mineral in thin section should be placed between crossed nicols of the geological polarizing light microscope, and the stage rotated until it occupies one of the positions where the polarization color is at its brightest. The mounted wedge should then be inserted into the slot of the eyepiece of the polarizing microscope until it occupies a position immediately above the fragment to be examined. In such circumstances the polarization effect is due partly to the mineral on the stage, and partly to the wedge in the slot. There are two possible cases. First, the effect of the two is as the sum of the two quantities having the same sign – the polarization colors of the two are higher in the scale than either alone. Second, the effect of the two is as the sum of two quantities having unlike signs – the polarization colors of the two are lower in the scale than either alone.

 

            It is the second case that we require. A rotation of the stage of the petrographic polarizing light microscope through 90 degree will convert case 1 into case 2. Having obtained the required position between section and wedge, it will be seen that at some position in the latter, a black band takes the place of one of the color-bands in the scale. The particular band of color replaced by this compensation band is the polarization color of the mineral plate. If the mineral plate whose polarization color we set out to determine is the only fragment in the slide, or if it is at the edge of a rock section, the fact may be demonstrated by placing the section in such a position that one side of the wedge only covers it. On this side, the compensation band will be obtained. On the other hand, the ordinary scale will be exhibited. And the compensation band will be continuous across the wedge with the color which is that exhibited by the section alone.

 

            The test may be applied to any anisotropic mineral section, though in the case of a small fragment, or one of irregular thickness, a little care is necessary in making the determination.



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Friday, February 15th, 2008 at 5:14 am
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Characters of Minerals Between Crossed Nicols
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