The Ruby.	151
	When Tavernier in his famous "Travels" describes the Ruby of Pegu, he says : " All other" coloured stones, in this country are called by the name Ruby, and are only distinguished by colour ; thus, in the language of Pegu, the Sapphire is a Blue Ruby." With reference to this passage, the late Prof. Ball, in his edition of the " Travels," remarks in a note ; " A very legitimate system. of nomenclature, as they are all of the same chemical com­position, viz ; alumina or corundum.'' Corundum, in all its varieties, crystallizes in the hexagonal system, usually in double six-sided pyramids, but often also in hexagonal prisms, and sometimes in six-sided plates or tabular crystals. The crystalline character of the Ruby furnishes, even in a cut stone, a ready means of distinguishing it from Garnet or from Spinel ; since the crystalline structure is closely correlated with certain optical properties. The use of the instrument called the dichroiscope renders the distinction a matter of certainty. This instrument enables us to see whether the gem possesses the property of dichroism—that is, of exhibiting two distinct colours, or tints, when viewed in different directions. Gems belonging to the Cubic system of crystallization do not exhibit this property, while in those belonging to any of the other systems this diversity may often be detected, when properly examined by the dichroiscope, be the stone ever so uniform in colour to the unassisted eye. Since both the Spinel and the Garnet belong to the Cubic or Tesserai system, they display no dichroism, whereas the Ruby, which belongs to the Hex­agonal system, is invariably dichroic. The typical Burma Ruby when examined by the dichroiscope, exhibits one image of crimson, while the other is an aurora-red colour.