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 



1^- Immersion Oil " 



cover Glass the objects appear bright or bril- 

 e 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 



FIG. 13. Dark-field Illumination. the reaSOn the field f W 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 Beck 1 dark-field 

 1 Made by R. & J. Beck, London. 



