CM. II.] 



LIGHTING AND FOCUSING. 



47 



§ 88. Dark-Ground Illumination. — When an object is lighted with 

 rays of a greater obliquity than can get into the front lens of the objec- 

 tive, the field will appear dark (Fig. 51 ). If now the object is cotn- 



48. 



49- 



50. 



51- 



Figs. 48-51. Sectional views of the Abbe Illuminator of 1.20 N. A. showing 

 various methods of illumination {\ 84). Fig. 4S, axial light with parallel rays. 

 Fig. 49, oblique light. Fig. 50, axial light with converging beam. Fig. 51, dark- 

 ground illumination with a central stop diaphragm. 



Axis. The optic axis of the illuminator and of the microscope. The illumina- 

 tor is centered, that is its optic axis is a prolongation of the optic axis of the 

 microscope. 



S. Axis. Secondary axis. In oblique light the central ray passes along a sec- 

 ondary axis of the illuminator, and is therefore oblique to the principal axis. 



D D. Diaphragms. These are placed in sectional and in face views. The dia- 

 phragm is placed between the mirror and the illuminator. In Fig. 49 the opening 

 is eccentric for oblique light, and in Fig. 5/ the opening is a narrow ting, the 

 central part being stopped out, and thus giving rise to dark-ground illumination 

 (188). 



Obj. Obj . The front of the objective. 



posed of fine particles, or is semi-transparent, it will refract or reflect 

 the light which meets it, in such a way that a part of the very oblique 

 rays will pass into the objective, hence as light reaches the objective 

 only from the object, all the surrounding field will be dark and the ob- 

 ject will appear like a self-luminous one on a dark back-ground. This 



