Petrographic Microscope

Petrographic microscope, also called a polarizing microscope, is best described as a compound transmitted light microscope to which components have been added to enable the determination of the optical properties of translucent substances. The designation of the microscope as a compound microscope indicates that it has an ocular that focuses on a virtual picture of the subject produced in the tube of the microscope by the objective lens. The substage of petrographic microscope that can be centered and focused has field and aperture diaphragms. The polarizing components are the upper and lower polarizing devices, the Bertrand lens and its mounting, between the upper polarizing device and the ocular, an accessory flip in lens for convergent light mounted as the top element of the condenser, and a graduated rotating stage with a removable click stop that can be activated to indicate a forty five degrees rotation from any selected direction. The focus knob and the stand are graduated to permit the determination of thickness by differential focusing.According to the plan of Nicol, the first polarizing devices were prisms fabricated from crystals of the mineral calcite specially cut and cemented back together. Such devices were originally called nicol prisms. Now they are called nicols. Crystals of calcite, CaC03, were chosen because light that goes through this substance in certain directions is split into two distinctly different rays strongly polarized at ninety degrees to each other that travel at widely different indices of refraction. Therefore, calcite has a high birefringence. Nicol used the plane on which he cemented the crystals back together as a plane of total reflection for the ray with the index of refraction most different from that of the cement.In modern petrographic microscopes, the nicols are polarizing plates fabricated in much the same way as are the lenses in polarized sunglasses. If two polarizing plates of sufficient thickness and quality are superimposed with their polarization directions at right angles to each other, no light can penetrate the pair. This is because the first polarizing plate excludes all light that is polarized perpendicular to the direction of the plate polarization, concomitantly polarizing the remaining light parallel to the plate polarization, and then the second plate does likewise. Together, all light is excluded. An indication of this effect can be observed by looking through two polarized sunglasses lenses superimposed at ninety degrees to each other. Polarized sunglasses work because all reflected light is, at least partially polarized parallel to the substance from which it is reflected and the polarizing material prevents the channel of rays polarized in a horizontal direction, such as that reflected from puddles, snow, automobile surfaces, and pavement. The effect may be observed if you look through a polarized lens at a patch of reflected glare while turning the lens to various orientations relative to the polarization of the glare. The sky is polarized by reflection from the molecules of the atmosphere.The petrographic microscope that was properly adjusted with nicols of sufficient quality, allows no light discernable to the eye to penetrate the upper and lower polarizing devices when their polarization directions are at 90 degrees to each other and there is no birefringent substance between them. However, if a birefringent substance, such as a crumpled piece of cellophane, is placed between two polarizing plates that are positioned with their polarization directions at ninety degrees to each other, the birefringence of the inner substance polarizes the light that travels through the first polarizing plate in directions parallel to the optical directions of the substance and the optical system will transmit light. The intensity and color of this transmitted light are controlled by the birefringence, optical orientation, and thickness of the interior substance. The lower and upper nicols, or polarizing plates, called the polarizer and analyzer, respectively, of the petrographic microscope act as do the plates of polarized material. Moreover, if both nicols are in the optical path and are oriented at right angles to each other and a birefringent specimen material is on the stage, the amount of the birefringence and many other optical properties may be determined. Original Text: