Water-chrysolite, 284 -sapphire (iolite), 266 -sapphire (topaz), 201 White opal, 249 White Saxon diamond, 165 Wollaston, W. H., 133 X-rays, 83 Yellow ground, 143 Zircon, 228 _Printed by_ MORRISON & GIBB LIMITED, _Edinburgh_ +--------------------------------------------------------------------+ | FOOTNOTES: | | | | [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] γωνία, angle; μέτρον, 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. | | | | [7] Cf. below, p. 149. | | | | [8] The least and the greatest of the refractive indices of | | doubly refractive species are given. | | |