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Ordinarily, a beam of light is vibrating in all directions. perpendicular to its line of travel (chelsea filter). A Polaroid acts as a selective filter, passing only that portion of the incident light which may be considered as vibrating in the plane of polarization .

Use of polariscope
In order to determine the optical characteristics of a gemstone in the gem testing polariscope, the analyzer is first turned to the dark position. The stone may be placed on the polarizer, as illustrated in Figure I or held either in a pair of tweezers or in the fingers between the polarizer and the analyzer.

An optic-axis direction is characterized in the polariscope's dark position by the presence of rainbow colors associated with an interference figure. The cause and analysis of interference figures by the polariscope is discussed in part B of the polariscope work later in the course. The observation of bright colors in a limited number of directions suggests optic axes and double refraction.

Biaxial stones seldom show any relation between the plane of the girdle and the optic axis.

The normal tendency for a person with little experience in the use of the polariscope is to drop any stone to be examined in the most convenient position (table down) and rotates it in that position. There is no particular objection to this, if several other positions are also used for the examination. The most serious danger concerns well-cut stones, particularly those that have a high R.I., such as diamond or zircon.