FIG. 89.—Black Central Band and equidistant Spectrum Bands on each side, afforded by Babinet’s Composite Plate of two Quartz Wedges, one parallel and the other perpendicular to the Axis. (Direct Photograph of Screen Picture as projected by Lantern Polariscope.) ] [Illustration: FIG. 94.—Section-plate of Amethyst, natural size, as seen directly in Ordinary Light, showing Alternate Violet Sectors (see p. 223). ] This effect of a black band with flanking spectra is very similar to that obtained, due to double refraction and not to optical activity, when two thin wedges of quartz are cemented together to form a parallel plate, one wedge being cut so that the optic axis is parallel to the edge of the wedge, and the other with the optic axis perpendicular to the edge. When such a composite plate of quartz, often known as a Babinet plate from the name of its first constructor, is placed on the stage of the polariscope, and rotated to the 45° position with respect to the planes of vibration of the crossed Nicols, there is observed on the screen a deep black band in the centre parallel to the edge of the wedge, and a number of spectrum bands on each side, separated by white equal interspaces, the rainbow coloured bands showing the orders of Newton’s spectra. The effect, as seen on the screen, is reproduced photographically in black and white in Fig. 89, Plate XVIII. These experiments lead us naturally to the study of a great variety of quartz twins, involving some of the most beautiful and gorgeously chromatic phenomena which it is possible to produce on the screen with the projection polariscope. They will eventually bring us to the study of amethyst quartz, in which the twinning is repeated so often that the laminations of alternate right and left quartz are sometimes countless, and almost approach molecular dimensions.