The =Greenough binocular microscope=, as shown in Fig. 19, consists of a microscope stage with two tubes mounted side by side and moving on the same rack and pinion for the focusing adjustment. Either tube can be used without the other. The oculars are capable of more or less separation to suit the eyes of different observers. In each of the drub-like mountings, near the point where the oculars are introduced, porro-prisms have been placed, which erect the image. This microscope gives most perfect stereoscopic images, which are erect instead of inverted, as in the monocular compound microscopes. The Greenough binocular microscope is especially adapted for dissection and for studying objects of considerable thickness. POLARIZATION MICROSCOPE The =polarization microscope= (Fig. 20) is used chiefly for the examination of crystals and mineral sections as well as for the observation of organic bodies in polarized light. It can, however, also be used for the examination of regular biological preparations. [Illustration: FIG. 20.--Polarization Microscope] If compared with the regular biological microscope, the polarization microscope is found characteristic of the following points: it is supplied with a polarization arrangement. The latter consists of a polarizer and analyzer. The polarizer is situated in a rotating mount beneath the condensing system. The microscope, of which the diagram is shown, possesses a triple “Ahrens” prism of calcite. The entering light is divided into two polarized parts, situated perpendicularly to each other. The so-called “ordinary” rays are reflected to one side by total reflection, which takes place on the inner cemented surface of the triple prism, allowing the so-called “extraordinary” rays to pass through the condenser. If the prism is adjusted to its focal point, it is so situated that the vibration plane of the extra-ordinary rays are in the same position as shown in the diagram of the illustration. The analyzer is mounted within the microscope-tube above the objective. Situated on a sliding plate, it can be shifted into the optical axis whenever necessary. The analyzer consists of a polarization prism after Glan-Thompson. The polarization plane of the active extraordinary rays is situated perpendicularly to the plane as shown in the diagram. The polarization prisms are ordinarily crossed. In this position the field of the microscope is darkened as long as no substance of a double refractive index has been introduced between the analyzer and polarizer. In rotating the polarizer up to the mark 90, the polarization prisms are mounted parallel and the field of the microscope is lighted again. Immediately above the analyzer and attached to the mounting of the analyzer a lens of a comparatively long focal length has been placed in order to overcome the difference in focus created by the introduction of the analyzer into the optical rays.