FIG. 22.—Olivine decomposed to serpentine. The pseudomorphism has been almost complete, only small portions of the original olivine remaining. The outline of the parent crystal can be quite distinctly seen. Crossed nicols. ] (_e_) _Pseudomorphic_ structure, which may be partial or complete and is noticed by the changed portions producing different optical effects from those of the original mineral. Sometimes, although the pseudomorphism has been almost complete, the form of the original mineral or crystal may still be seen, Fig. 22. Characters Observed by Convergent Polarized Light. _Convergent light_ is obtained by passing the rays of polarized light through a strong condensing lens, which generally fits like a cap over the top of the polarizer. By means of a suitable adjustment the condensing lens can be brought very close to the lower surface of the section on the stage. The lens thus sends a cone of light through the section, and used in connection with _crossed nicols_ a series of optical phenomena, called _interference figures_,[68] are produced. Each direction in which rays are sent is traversed by a minute bundle of parallel rays and these rays extinguish and produce interference colors as already described for parallel light. Hence each direction yields a spot or picture in the field of view and from all these spots combined there results an “interference figure” or picture, depending upon the structure of the section for all the directions traversed by the rays. A very high power objective[69] must be used, and when the eye-piece is removed, a small image of the interference figure will be seen. In some microscopes an arrangement is made for getting a magnified image of the interference figure, by retaining the eye-piece and using an additional Bertrand lens. In order to get good results care must be taken to have strong illumination and the condensing lens close up under the section. The tests are best made with monochromatic light, but with white light the effects are substantially the same, the only difference being that the rings and curves are variously colored instead of being simply light and dark. _Isotropic_ substances show no _interference figures_. Uniaxial Interference Figures. (_a_) Sections perpendicular to the optic or vertical axis _ć_ show a dark cross, with or without colored rings, Figs. 23 and 24. The figure is symmetrical to the center, as the optical behavior of uniaxial crystals is symmetrical to the optic axis. [Illustration: FIG. 23. ] [Illustration: FIG. 24. ] The arms of the cross are parallel to the planes of vibration of the nicols, and the figure does not move when the stage carrying the section is rotated.[70]