Abstract
By means of a relatively simple mechanical device, called an isogyrometer, it is possible to show students of optical mineralogy why isogyres exist, why they behave as they do, and how the positions of the optic elements can be determined from interference figures. The isogyrometer consists of a box-frame supporting a rotatable sphere that can be viewed through a circular window representing the microscope field. Lines of equal velocity (isotaques) are drawn on the sphere, and lines representing the vibration directions of the crossed polarizers in a microscope are drawn on the window. Where the isotaques and the lines on the window are parallel, light would be extinguished under the microscope, producing isogyres. The device includes four interchangeable spheres, one to represent uniaxial minerals and the others to represent biaxial minerals with optic angles of 30°, 60°, and 90°. Construction of the isogyrometer is explained, and its uses and limitations are discussed.
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