. Essentials of laboratory diagnosis; designed for students and practitioners. gles to the axis, oblique lightmay be obtained through the condenser, as follows: (b) First,focus the light upon the object through the condenser; then,lower the condenser until its focus is considerably below theplane of the object; thus the rays emerging from the condenserwill decussate and then diverge, so that all but axial rays willfall upon the object in ah oblique direction. DARK-GROUND ILLUMINATION. DARK-GROUND ILLUMINATION. The Substage Dark-ground Illuminator.—Most high-grademicroscopes are now fitted with
. Essentials of laboratory diagnosis; designed for students and practitioners. gles to the axis, oblique lightmay be obtained through the condenser, as follows: (b) First,focus the light upon the object through the condenser; then,lower the condenser until its focus is considerably below theplane of the object; thus the rays emerging from the condenserwill decussate and then diverge, so that all but axial rays willfall upon the object in ah oblique direction. DARK-GROUND ILLUMINATION. DARK-GROUND ILLUMINATION. The Substage Dark-ground Illuminator.—Most high-grademicroscopes are now fitted with a substage dark-ground con-denser. (See Fig. 2.) If this is available, the microscope shouldbe set in a vertical position and the dark-ground condenser sub--i ituted for the regular Abbe. If daylight is used, no parallelingdevice is necessary. If artificial light is used a bulls-eye con-denser must be interposed between the substage reflector andthe light, in such a way that parallel rays strike and completelycover the plain substage mirror. Then focus the two concentric. Fig. 2.—Dark-field Illuminator. rings engraved upon the upper surface of the condenser andcenter them accurately by means of the screws provided for thispurpose. Preparation of Fresh Specimen.—When employing the dark-field condenser the material to be examined should be in a fresh,moist state, if the best results are to be obtained. Carefullysmear the specimen to be examined upon one or more slides,preferably new ones that have not been scratched by handling,and cover with thoroughly cleansed new No. 1 a large drop of immersion-oil upon the upper surface ofthe condenser and then fasten the specimen on the microscopestage, bringing the condenser up until the immersion-oil is incontact with the under surface of the slide. A properly adjustedmirror should show a bright spot in the center of the mountedslide. Place a drop of cedar oil in the center of the cover-glassand bring the interme
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