STAGE AND SUB-STAGE CONDENSERS, DIAPHRAGMS, ETC.
The bull's-eye condenser cannot be used alone for illumination when high powers are employed : for these some form or stage or substage condenser has to be employed. The stage condenser (Fig. 703) is mounted in a sleeve which slides into the tube below the stage ; the substage con forms, one with a numerical aperture of i.2 (Fig. 701), the other of (Fig. 705), the amount of light collected by the latter being proportionately greater. Fig. 706 shows the " pan-aplanatic " condenser of Swift, N.A. : Fig. 707, a universal con denser (holoscopie system) by Watson ; Fig. 708, an apochromatic condenser ; and Fig. 709, an oil-immersion condenser by Watson. All these condensers are suit able for use with objectives of high mag nifying powers. In achromatic condenser is much superior to an ordinary condenser, denser is the same as far as the optical part goes, but it is fitted into the sub stage. These condensers are either non achromatic, achromatic, or apochromatic. The non-achromatic condensers are com posed of two lenses, while the achromatic and apoehromatic condensers are made up of various combinations. A well-known form of achromatic condenser is the Abbe illuminator, which is made in two as the light which it transmits is prac tically free from colour. The oil-immer
sion condenser is used with an oil-im mersion objective, a drop of oil of cedar wood being placed on the front lens, and the condenser racked up until it touches the under surface of the glass slide on which the object is mounted ; the light then .passes in a straight path from the spot lens is simply a plano-convex lens, upon which is painted a circle with black varnish, which prevents the central rays reaching the objective. But by far the best dark-ground illumination is obtained by means of the paraboloid (Fig. 713), a lens hollowed in the front, and in which is fixed a curved piece of blackened metal, which may be raised or lowered condenser to the objective. The densers are provided with interchangeable diaphragms of various sizes for use with the different objectives (Fig. 710). These are simply metal discs perforated with holes of various sizes in the centres ; but the iris diaphragm (Fig. 711) has now almost superseded the older form, because it can be regulated to any diameter of opening by simply revolving the outer ring Ivhile the object is in focus, and in this way the illumination may be adjusted to show the greatest amount of detail.