The application of the polarising apparatus to the microscope is of much value in determining minute structure. It may also be defined as an instrument of analysis; a test of difference in density between any two or more parts of the same substance. All structures, therefore, belonging either to the animal, vegetable, or mineral kingdom, in which the power of unequal or double refraction is suspected to be present, are those that should especially be re-investigated by polarised light. Some of the most delicate of the elementary tissues of animal structure, the ultimate fibrillæ of muscles, &c., are amongst the most interesting subjects that might be studied with advantage under this method of investigation. The chemist may perform the most dexterous analysis; the crystallographer may examine crystals by the nicest determination of their forms and cleavage; the anatomist or botanist may use the dissecting knife and microscope with the most exquisite skill; but there are still structures in the mineral, vegetable, and animal kingdoms which will defy all such modes of examination, and will yield only to the magical analysis of polarised light.

Formation and Polarisation of Crystals.

The inorganic kingdom will afford to the microscopist a never-ending number of objects of unsurpassed beauty and interest. The phenomena of crystallisation in its varied combinations can be made a useful and instructive occupation. Although ignorant of the means whereby the great majority of minerals and crystals have been formed in the vast laboratory of Nature, we can, nevertheless, imitate in a small degree Nature’s handiworks by crystallising out a large number of substances, and watch their numerous transformations in the smallest appreciable quantities, when aided by the microscope.

Among natural crystals we look for the material for the formation of our lenses, while the varieties of granites present us with the earliest crystallised condition of the earth’s crust as it cooled down, the structure of which is beautifully exhibited under polarised light. In [Plate VIII]. various crystalline and other bodies are displayed. In No. 158 is a section of new red sandstone; 159 of quartz; and 160 of granite. Special reference is made to others in the following list of salts and other substances which form a beautiful series of objects for study under polarised light:—

SALTS.

Alum.
Asparagine.
Aspartic Acid. [Plate VIII]. No. 168.
Bitartrate of Ammonia.
Boracic Acid.
Borax. No. 164.
Carbonate of Lime.
" Soda.
Chlorate of Potash.
Chloride of Barium.
" Cobalt.
" Copper and Ammonia.
" Sodium.
Cholesterine.
Chromate of Potash.
Cinchonine.
Cinchonidine.
Citric Acid.
Hippuric Acid.
Iodide of Mercury.
" Potassium.
" Quinine.
Iodo-disulphate of Quinine.
Kreatine. No. 166.
Murexide.
Nitrate of Bismuth.
" Barytes.
" Brucine.
" Copper.
" Potash.
" Strontian.
" Uranium.
Oxalate of Ammonia.
" Chromium.
" Chromium and Potash.
" Lime.
" Soda.
Indurated Sandstone, Howth.
Indurated Sandstone, Bromsgrove.
Gibraltar Rock.
Granite, various localities. No. 160.
Hornblend Schist.
Labrador Spar.
Norway Rock.
Quartz Rock, various. No. 159.
" in Bog Iron Ore.
Quartzite, Mont Blanc.
Sandstone. No. 158.
Satin Spar.
Selenites, various colours.
Tin Ore, with Tourmalin.
Oxalic Acid.
Oxalurate of Ammonia.
Permanganate of Potash.
Phosphate of Lead and Soda.
Platino-cyanide of Magnesia.
Plumose Quinidine.
Prussiate of Potash, red and yellow.
Quinidine.
Santonine.
Salicine.
Salignine. No. 162.
Sulphate of Cadmium.
" Copper. No. 161.
" Copper and Potash.
" Iron. No. 163.
" Iron and Cobalt. No. 165.
" Magnesia.
" Nickel and Potash.
" Soda.
" Zinc.
Sugar.
Tartaric Acid.
Thionurate of Ammonia.
Triple Phosphate.
Urate of Ammonia.
" Soda.
Urea, and most urinary deposits.
Uric Acid.

MINERALS.

Agates, various.
Asbestiform Serpentine.
Avanturine.
Carbonate of Lime.
Carrara Marble.

ANIMAL STRUCTURES.

Cat’s Tongue. No. 174.
Grayling Scale. No. 176.
Holothuria, Spicules of. Nos. 171-2.
Prawn Shell. No. 175.