. Elementary chemical microscopy. Microscopy; Microchemistry. 38 ELEMENTARY CHEMICAL MICROSCOPY reflecting condenser and the preparation on the stage, a drop or two of homogenous immersion fluid is placed between con- denser and object slide. If, however, an object lies in the path of the rays, refraction, reflection and diffraction take place and the object becomes brightly illuminated, or if submicro- scopic particles are in suspension in the medium between object slide and cover glass diffraction patterns result and appear to the eye as brilliant points of light surrounded by more or less d
. Elementary chemical microscopy. Microscopy; Microchemistry. 38 ELEMENTARY CHEMICAL MICROSCOPY reflecting condenser and the preparation on the stage, a drop or two of homogenous immersion fluid is placed between con- denser and object slide. If, however, an object lies in the path of the rays, refraction, reflection and diffraction take place and the object becomes brightly illuminated, or if submicro- scopic particles are in suspension in the medium between object slide and cover glass diffraction patterns result and appear to the eye as brilliant points of light surrounded by more or less distinct alternate bright and dark rings. These points of light exhibit rapid vibratory motions (Brownian movement). To prevent axial light from passing through the illuminator an opaque stop is placed in the optic axis of the device. The field is therefore black or nearly so, save for a slight halo at its edges, while the objects appear bright or bril- pbjective Immersion Oil Cover Glass Slide Immersion. Fig. 13. Dark-field Illumination. on liantly colored upon a dark back- ground. In Fig. 13 a simple paraboloid reflecting illuminator is shown diagrammatically in section, with the directions of the light rays so exaggerated as to make clearer the reason the field of view is dark. Sections of typical illuminators are shown in Fig. 14, A, B, C, D. It will be seen that although the construction may be different in different types, the rays emerge at approximately similar angles. In illuminators of thesr types (B, C, D) the curvatures of the reflecting surfaces are ground after mathematically calculated curves which will bring the light rays approximately to a focus at a point just at the upper surface of the slide or slightly above this plane. In the diagrams for simplicity, cover glasses and preparations have been omitted. An exception to the above statement, relative to the construc- tion of reflecting condensers, is found in the Beck1 dark-field 1 Made by R. & J. Beck, Lond
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