Many transparent solids are optically isotropic, meaning that the index of refraction is equal in all directions throughout the crystalline lattice. Examples of isotropic solids include glass, table salt (sodium chloride), many polymers, and a wide variety of both organic and inorganic compounds.

Crystals are classified as being either isotropic or anisotropic depending upon their optical behavior and whether or not their crystallographic axes are equivalent. All isotropic crystals have equivalent axes that interact with light in a similar manner, regardless of the crystal orientation with respect to incident light waves. Light entering an isotropic crystal is refracted at a constant angle and passes through the crystal at a single velocity without being polarized by interaction with the electronic components of the crystalline lattice.

Anisotropic crystals, on the other hand, have crystallographically distinct axes and interact with light in a manner that is dependent upon the orientation of the crystalline lattice with respect to incident light. When light enters the optical axis of anisotropic crystals, it acts in a manner similar to interaction with isotropic crystals and passes through at a single velocity. However, when light enters a non-equivalent axis, it is refracted into two rays each plane-polarized with their vibration directions oriented at right angles to one another, and traveling at different velocities. This phenomenon is termed double or bi-refraction and is seen to a greater or lesser degree in all anisotropic crystals. This gallery features digital images taken with a QX3 microscope that has been modified for crossed-polarized illumination (see Figure 1) with an advanced achromatic substage condenser.

Atropine - Derived from plants such as belladonna and henbane, this alkaloid has a variety of medical uses. Atropine is an anticholinergic agent that is a potent parasympatholytic, inhibiting actions of acetylcholine at postganglionic parasympathetic neuroeffector sites. It is a competitive antagonist of acetylcholine at smooth and cardiac muscles and various glandular cells.

Biotin (Vitamin H) - Biotin is a water-soluble member of the B-complex group of vitamins and is commonly referred to as vitamin H. Crystallites of biotin display marked enhancement of image detail and sharpness when illuminated with an advanced substage condenser.

Citric Acid - This substance is a nearly universal intermediate product of metabolism in plants and animals and has many commercial uses. Citric acid is a colorless organic tri-carboxylic acid that is commonly found in citrus fruits such as lemons, limes, and oranges. In biochemical intermediary metabolism, citric acid is heavily involved in the tricarboxylic acid or Krebs cycle that occurs in all plants and animals as well as most bacteria.

DDT - The world's best known pesticide, DDT, is a deadly chemical that has been banned in many countries, including the United States. Digital images of recrystallized DDT taken with the QX3 microscope reveal the beautiful spherulites formed by this dangerous chemical.

Dihydroxy Pyrimidine - Otherwise known as uracil, this compound is a building block for RNA, the hereditary material of viruses and the protein making machinery of the cell.

Menthol - Menthol is the principal constituent of peppermint oil, and can be isolated as a pure white crystalline substance. The fragrance and flavor qualities of this organic chemical have been exploited for many years. Menthol produces a cool sensation when tasted and smelled, and is commercially found in lotions, shaving creams, and cough drops.

Pyridoxine (Vitamin B-6) - This coenzyme is remarkably versatile, being involved in transaminations, decarboxylations, racemizations, and numerous modifications of amino acid side chains. Clinically, pyridoxine helps normal function of the brain, promotes blood cell formation, maintains the chemical balance among body fluids, and assists in carbohydrate, protein, and fat metabolism.

Silver Nitrate - The nitrate salt of silver is one of the most useful salts from this precious metal. Silver nitrate is used in photography, the manufacturing of mirrors, silver plating, indelible inks, hair dyeing, coloring ceramics, and etching ivory. Enhanced image sharpness and resolution are quite evident in these polarized light digital images of silver nitrate when an auxiliary condenser is used to control illumination.

TNT - The chemical is prepared by carefully nitrating toluene, and is often used as a high explosive. Unlike nitroglycerin, trinitrotoluene does not explode when affected by ordinary shocks and jarring, and must be ignited with a detonation cap.

Urea - Excess nitrogen in the body is excreted in one of three forms: ammonia (as the ammonium ion), urea, and uric acid. Animals, such as fish, that live in the water excrete nitrogen as ammonia, which is quickly diluted by the aqueous environment. In terrestrial animals, the primary waste product of nitrogen metabolism is urea, a water-soluble compound.

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