Another property which may be made use of in deciding the identity of certain gems is that called _dispersion_. We have seen in Lesson II. that light in entering a stone from the air changes its path (refraction), and in Lesson III. it was explained that many minerals cause light that enters them, to divide and proceed along two different paths (double refraction). Now it is further true that light of the various colors (red, orange, yellow, green, blue, and violet) is refracted variously--the violet being bent most sharply, the red least, and the other colors to intermediate degrees. The cut (Fig. 7) represents roughly and in an exaggerated manner the effect we are discussing. [Illustration: FIG. 7.] Now in a cut stone this separation of light of different colors, or dispersion of light, as it is called, results in the reflection of each of the colors separately from the steep sloping back facets of the stone. If almost any clear, colorless facetted stone is placed in the sunlight and a card held before it to receive the reflections, it will be seen that rainbow-like reflections appear on the card. These _spectra_, as they are called, are caused by the dispersion of light. With a diamond the spectra will be very brilliant and of vivid coloring, and the red will be widely separated from the blue. With white sapphire or white topaz, or with rock crystal (quartz), the spectra will be less vivid--they will appear in pairs (due to the double refraction of these minerals), and the red and blue will be near together (_i. e._, the spectra will be short). This shortness in the latter cases is due to the small dispersive power of the three minerals mentioned. Paste (lead glass) gives fairly vivid spectra, and they are single like those from diamond, as glass is singly refracting. The dispersion of the heavy lead glass approaches that of diamond. The decolorized zircon (jargoon) has a dispersion well up toward that of diamond and gives fairly vivid spectra on a card, but they are double, as zircon is doubly refracting. Sphene (a gem rarely seen in the trade) and the demantoid garnet (a green gem often called "olivine" in the trade) both have very high dispersive power, exceeding the diamond in this respect. As they are both colored stones (sphene is usually yellowish, sometimes greenish or brown), the vividness of their color-play is much diminished by absorption of light within them. So also the color-play of a deeply colored fancy diamond is diminished by absorption.