The longer the tube the lower the magnification. For instance, a two-inch ocular magnifies less than an inch and a half, a one-inch less than a three-fourths of an inch, etc.
The greater the curvature of the lenses of the ocular the higher will be the magnification and the shorter the tube-length.
FORMS OF COMPOUND MICROSCOPES
The following descriptions refer to three different models of compound microscopes: one which is used chiefly as a pharmacognostic microscope, one as a research microscope stand, while the third type represents a research microscope stand of highest order, which is used at the same time for taking microphotographs.
Fig. 16.—Pharmacognostic Microscope
PHARMACOGNOSTIC MICROSCOPE
The pharmacognostic microscope (Fig. 16) is an instrument which embodies only those parts which are most essential for the examination of powdered drugs, bacteria, and urinary sediments. This microscope is provided with a stage of the dimensions 105 × 105 mm. This factor and the distance of 80 mm. from the optical centre to the handle arm render it available for the examination of even very large objects and preparations, or preparations suspended in glass dishes. The stand is furnished with a side micrometer, a fine adjustment having knobs on both sides, thereby permitting the manipulation of the micrometer screw either by left or right hand. The illuminating apparatus consists of the Abbé condenser of numerical aperture of 1.20, to which is attached an iris diaphragm for the proper adjustment of the light. A worm screw, mounted in connection with the condenser, serves for the raising and lowering of the condenser, so that the cone of illuminating pencils can be arranged in accordance to the objective employed and to the preparation under observation. The objectives necessary are those of the achromatic type, possessing a focal length of 16.2 mm. and 3 mm. Oculars which render the best results in regard to magnification in connection with the two objectives mentioned are the Huyghenian eye-pieces II and IV so that magnifications are obtained varying from 62 to 625. It is advisable, however, to have the microscope equipped with a triple revolving nose-piece for the objectives, so that provision is made for the 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.