  [1] The word medium is employed by physicists to express any substance through which light passes, and includes solids such as glass, liquids such as water, and gases such as air; the nature of the substance is not postulated. [2] Methylene iodide must be heated almost to boiling-point to enable it to absorb sufficient sulphur; but caution must be exercised in the operation to prevent the liquid boiling over and catching fire, the resulting fumes being far from pleasant. It is advisable to verify by actual observation that the liquid is refractive enough not to show any shadow-edge in the field of view of the refractometer. [3] ?????, angle; ??????, measure. For details of the construction, adjustment, and use of this instrument the reader should refer to textbooks of mineralogy or crystallography. [4] A cleavage flake of topaz may conveniently be used to show the phenomenon, but owing to the great width of the angle the “eyes” are invisible. [5] In accordance with the usual custom the angle between the facets is taken as that between their normals, or the supplement of the salient angle. [6] The word paste is derived from the Italian, pasta, food, being suggested by the soft plastic nature of the material used to imitate gems. [8] The least and the greatest of the refractive indices of doubly refractive species are given. [9] The dispersion is the difference of the refractive indices corresponding to the B and G lines of the solar spectrum. The value for crown-glass is ·016. INTERESTING AND IMPORTANT BOOKS JEWELLERY. By Cyril Davenport, F.S.A. With a Frontispiece in Colour and 41 other Illustrations. Second Edition. Demy 16mo.[Little Books on Art. JEWELLERY. By H. Clifford Smith, M.A. With 50 Plates in Collotype, 4 in Colour, and 33 Illustrations in the text. Second Edition. Wide royal 8vo, gilt top.[Connoisseur’s Library. GOLDSMITHS’ AND SILVERSMITHS’ WORK. By Nelson Dawson. With 51 Plates in Collotype, a Frontispiece in Photogravure, and numerous Illustrations in the text. Second Edition. Wide royal 8vo, gilt top.[Connoisseur’s Library. EUROPEAN ENAMELS. By H. H. Cunynghame, C.B. With 58 Illustrations in Collotype and Half-tone and 4 Plates in Colour. Wide royal 8vo, gilt top.[Connoisseur’s Library. ENAMELS. By Mrs. Nelson Dawson. With 33 Illustrations. Second Edition. Demy 16mo.[Little Books on Art. Transcriber’s Notes:
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