Assuming that the polarizers are crossed to produce a dark field, the polariscope is then described as a circular dark-field polariscope. the polariscope is changing from a dark-field configuration to a light-field configuration. Photoelasticity is a nondestructive, whole-field, . the polariscope must be arranged so as to allow light .. izer always looks dark because half the light striking. A polariscope uses polarized light for gem identification. is at right angles to the vibrational direction of the analyzer, the field between them remains dark. Throughout a ° rotation the stone blinks 4 times, light and dark.
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In the direction marked “slow”, the slow ray of the wave plate travels. Although one can get reasonably nice images with them, a conoscope rod is preferred and the smaller the sphere, the sharper the image. With the polarizer and analyzer in crossed position, turn on the light source and place the gemstone on the rotating platform just above the polarizer this platform might not always be present, in which case you use your tweezers.
One just above the lightsource and the other just below the optics. Retardation plates as those added minerals are known have a known retardation, and the vibrational directions of the slow and fast rays are known. The wave plate creates yellow 1st order colors on the convex sides and 2nd order blue on the concave sides of the isogyres. The interference figures always appear around the optic axes of minerals.
Inserted full wave plate creates blue colors on the convex sides of the isogyres and yellow on the concave sides. Florists usually have cellophane plastic in which they wrap their flowers. It is also the preferred tool — next to the microscope — for separating synthetic amethyst from its natural counterparts although with recent synthetics that may prove difficult. Remember that uniaxial minerals have one optic axis while biaxial gemstones have two optical axes.
With the quarter wave inserted the Airy Spirals spiral to the left. Determining the optic sign in anisotropic gemstones should pose few problems with the aid of one of the retardation plates. When the slow ray of the gemstone and the slow ray of the added mineral align, the shift will be to the right. Views Read View source View history. When the vibrational plane of the polarizer is at right angles to the vibrational direction of the analyzer, the field between them remains dark.
One must now pay good attention to how the isochromes the colored rings behave. If you now move the wedge back from quadrant 1 to 3, the reverse is observed.
The distance between the two melatopes is dependent on the “2V” value of the mineral. Many polariscopes for gemological purposes come with a rather large conoscope that can be swivelled like a gemstone holder.
Place the stone is a shallow dish of water or baby oil and rotate the stone slowly lighr it. Simply lay one of the sheets over the transmitting lightsource and tape the other one in crossed position below the optics or find your own way of doing something similar.
With a stone of known optic sign you can determine that yourself though. Older models were created with microscopically oriented crystals of iodoquinine sulfate herapathite or tourmaline plates. The first 3 behaviors should pose no problems for the polariecope user, but the latter ADR can be misinterpreted and cause one to think the stone is double refractive.
Polariscope – The Gemology Project
Because anisotropic minerals appear to be single refractive when viewed down the optic axis, another technique for finding the optic axis can be used.
Observing the gemstone through the analyzer while slowly turning the stone will give you 4 possibilities. Same image as above, but enlarged and the waveplate removed for better view for illustration purposes only. This also depends on the “numerical aperture” of your microscope. The wedge is not used as such mostly in gemology, instead it is used to hover over an interference pattern and to determine optic sign by observing the movement of the isochromes.
When the slow ray leaves the gem, the fast ray would have already traveled an extra distance outside the gemstone. Here the curvature is easily seen and from that we know where the other melatope is located. Through a series of calculations, it is shown that this retardation is dependent on the thickness and birefringence of the gemstone. With the addition of a quarter wave plate this bull’s-eye transforms in two spirals which spiral either to the left or right, showing their “handedness”.
In most cases you will see only one isogyre and that is all we need for the quartz wedge to work. In addition, the polariscope may be very useful for distinguishing solid inclusions from negative inclusions as well as for spotting polysynthetic twinning. This is caused by the magenta color of the full wave plate under crossed polars the color in natural daylight is transparent white.
Here the isochromes in the 1st and 3rd quadrants will move out and the isochromes in the 2nd and 4th will move inward.
This technique works best with magnification as the obtained figures are very drk. When the slow ray of the gemstone and the fast ray of the added mineral align, the shift will be to the left and will create a subtraction in color. With monochromatic lighting such as a yellow sodium filter of your refractometerthe interference pattern may stand out more clearly.
Fortunately, modern day technology has created anisotropic plastic substitutes that cost little and can be held between your fingers. Some quartz especially amethyst is lgiht right-handed as left-handed due to Brazil twinning.
Red stones that are out of the limit of the refractometer OTL may be especially difficult to distinguish with the polariscope due to ADR. A few hours of practise should be enough to master this technique and it may come in very handy when you can perform little other tests. Because there may be more than one direction in which some gemstones remain dark, it is useful as a confirmation to view the stone under a different angle when it stays dark. With practice you will be able to recognize it more rapidly.
This position is known as the “crossed position”. Here the interference pattern has the isogyres in the lower right and upper left, but they could be in the lower left and upperright aswell. Both the polarizer and the analyzer have their own vibrational planes. Hover it back and forth over the interference pattern, but pay attention only to the change in the forward direction. For convenience, the image at the left has the area of interest marked, which is the area just around the center of the interference figure the white circle.
This cellophane may work as a quarter wave plate. Using an immersion cell along with the polariscope may enable you to find the flash figures more rapidly.