be moved on the plane surface of the dense glass until the greatest possible distinctness is imparted to the edge or edges. If it be moved towards the observer from the further end, a misty shadow appears to move down the scale until the correct position is reached, when the edges spring into view. Any facet of a stone may be utilized so long as it is flat, but the table-facet is the most convenient, because it is usually the largest, and it is available even when the stone is mounted. That the stone need not be removed from its setting is one of the great advantages of this method. The smaller the stone the more difficult it is to manipulate; caution especially must be exercised that it be not tilted, not only because the shadow-edge would be shifted from its true position and an erroneous value of the refractive index obtained, but also because a corner or edge of the stone would inevitably scratch the glass of the instrument, which is far softer than the hard gem-stones. Methylene iodide will in time attack and stain the glass, and must therefore be wiped off the instrument immediately after use. (2) THE METHOD OF MINIMUM DEVIATION If the stone be too highly refractive for a measurement of its refractive index to be possible with the refractometer just described, and it is desired to determine this constant, recourse must be had to the prismatic method, for which purpose an instrument known as a goniometer[3] is required. Two angles must be measured; one the interior angle included between a suitable pair of facets, and the other the minimum amount of the deviation produced by the pair upon a beam of light traversing them. [Illustration: FIG. 22.—Path at Minimum Deviation of a Ray traversing a Prism formed of two Facets of a Cut Stone.] Fig. 22 represents a section of a step-cut stone perpendicular to a series of facets with parallel edges; _t_ is the table, and _a, b, c_, are facets on the culet side. The path of light traversing the prism formed by the pair of facets, _t_ and _b_, is indicated. Suppose that _A_ is the interior angle of the prism, _i_ the angle of incidence of light at the first facet and _i´_ the angle of emergence at the second facet, and _r_ and _r´_ the angles inside the stone at the two facets respectively. Then at the first facet light has been bent through an angle _i - r_, and again at the second facet through an angle _i´ - r´_; the angle of deviation, _D_, is therefore given by _D = i + i´ - (r + r´)_. We have further that _r + r´ = A_, whence it follows that _A + D = i + i´._