When, instead of employing monochromatic flames, the spectroscopic monochromatic illuminator (Fig. 75, page 193), described by the author some years ago to the Royal Society, is employed to illuminate the polariscope, the source of light being the electric arc, the change of the figure from that given by the extreme red of the spectrum to that afforded by the violet may be beautifully followed, and the exact wave-length in the greenish yellow determined for which the crossing occurs and an apparently uniaxial figure of circular rings and rectangular cross is produced. For it is possible with the aid of this illuminator directly to observe the production of the uniaxial figure. The wave-length is either directly afforded by the graduation of the fine-adjustment micrometric drum or is obtained from a curve of wave-lengths, constructed to correspond to the circle readings of the illuminator. The appearance of the interference figure for this critical wave-length is shown at _c_ in Fig. 58. The remaining figure at _e_ represents the appearance when a mixture of sodium and thallium light is employed, which clearly indicates the four extreme axial positions, and assists in elucidating the nature of the figure _f_ exhibited in white light. The second form of the propyl derivative belongs to the rhombic system, and a similar rhombic form of the ethyl compound was once obtained, but lost again on attempting to recrystallise. These interesting relationships of the homologous methyl, ethyl, and propyl derivatives of triphenyl pyrrholone thus appear to form a connecting link between cases of isogonism or morphotropy and of true isomorphism. [Illustration: _PLATE XIII._ FIG. 58.—Interference Figures in Convergent Polarised Light of different Wave-lengths afforded by the Monoclinic Variety of Ethyl Triphenyl Pyrrholone; _a_, in Red Lithium Light; _b_, in Yellow Sodium Light; _c_, in Greenish-Yellow Light of the Critical Wave-length for Production of the Uniaxial Figure; _d_, in Green Thallium Light; _e_, in mixed Sodium and Thallium Light; and _f_, in White Light. (Reproductions of Drawings by the author.) ]