addition of an oil-immersion objective at any time later should the microscope become available for bacteriological investigations. THE RESEARCH MICROSCOPE The =research microscope= used in research work (Fig. 17) must be equipped more elaborately than the microscope especially designed for the use of the pharmacognosist. While the simple form of microscope is supplied with the small type of Abbé condenser, the research microscope is furnished with a large illuminating apparatus of which the iris diaphragm is mounted on a rack and pinion, allowing displacement obliquely to the optical centre, also to increase resolving power in the objectives when observing those objects which cannot be revealed to the best advantage with central illumination. Another iris is furnished above the condenser; this iris becomes available the instant an object is to be observed without the aid of the condenser, in which case the upper iris diaphragm allows proper adjustment of the light. The mirror, one side plane, the other concave, is mounted on a movable bar, along which it can be slid--another convenience for the adjustment of the light. The microscope stage of this stand is of the round, rotating and centring pattern, which permits a limited motion to the object slide: The rotation of the microscope stage furnishes another convenience in the examination of objects in polarized light, allowing the preparation to be rotated in order to distinguish the polarization properties of the objects under observation. [Illustration: FIG. 17.--Research Microscope] [Illustration: FIG. 18.--Special Research Microscope] SPECIAL RESEARCH MICROSCOPE A =special research microscope= of the highest order (Fig. 18) is supplied with an extra large body tube, which renders it of special advantage for micro-photography. Otherwise in its mechanical equipment it resembles very closely the medium-sized research microscope stand, with the exception that the stand is larger in its design, therefore offering universal application. In regard to the illuminating apparatus, it is advisable to mention that the one in the large research microscope stand is furnished with a three-lens condenser of a numerical aperture of 1.40, while the medium-sized research stand is provided with a two-lens condenser of a numerical aperture of 1.20. The stage of the microscope is provided with a cross motion--the backward and forward motion of the preparation is secured by rack and pinion, while the side motion is controlled by a micrometric worm screw. In cases where large preparations are to be photographed, the draw-tube with ocular and the slider in which the draw-tubes glide are removed to allow the full aperture of wide-angle objectives to be made use of. [Illustration: FIG. 19.--Greenough Binocular Microscope] BINOCULAR MICROSCOPE