Elementary chemical microscopy (1921) Elementary chemical microscopy elementarychemi00cham Year: 1921 OPTICAL PROPERTIES OF CRYSTALS 253 to their characteristic morphology, crystals exhibit certain physi- cal and optical properties according to the crystal system to which they are referred. Chief among these optical properties made use of by the chemist is the behavior of the crystals towards polarized light. Optically, crystals are either singly refractive (isotropic) or doubly refractive (anisotropic). If isotropic, they will show no change when rotated upon the stage of the microscope betw
Elementary chemical microscopy (1921) Elementary chemical microscopy elementarychemi00cham Year: 1921 OPTICAL PROPERTIES OF CRYSTALS 253 to their characteristic morphology, crystals exhibit certain physi- cal and optical properties according to the crystal system to which they are referred. Chief among these optical properties made use of by the chemist is the behavior of the crystals towards polarized light. Optically, crystals are either singly refractive (isotropic) or doubly refractive (anisotropic). If isotropic, they will show no change when rotated upon the stage of the microscope between crossed nicols. If anisotropic, they will appear alternately light and dark as the stage is turned. If, therefore, a crystal be placed upon the stage of a polarizing microscope near the center of the field between crossed nicols and the stage turned, the crystal will behave in one of two ways: i. It will remain dark throughout a complete rotation of the stage, that is, there is no change in its appearance in the dark field. 2. As the stage is turned the crystal will alternately become bright or colored, and alternately disappear or become dark (ex- tinguish). In this case two pos- sibilities arise. Either the crys- tal disappears (extinguishes) when its long edges coincide with or are parallel to the cross-hairs, and is brightest midway between, or the position of extinction is not on the cross-hairs, but lies a little inclined to (is oblique to) the cross-hairs. In the former Fig. 142. Isotropic and Anisotropic Crystals between Crossed Nicol Prisms.
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