CH. IT] 



LIGHTING AND FOCUSING 



49 



with reference to the illuminating ray ? What in focusing up ? If one 

 understands the experiment it may sometimes save a great deal of con- 

 fusion. (See under testing the microscope for swaying with central 

 light § 119.) 



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

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

 objective, the field will appear dark (Fig. 51). If now the object is 



Figs. 48-51. Sectional views of the Abbe Illuminator of 1 .20 N. A. showing 

 various methods of illumination (§ qo). Fig. 48, 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 illumi- 

 nator 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 

 secondary 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 

 diaphragm is placed between the mirror and the illuminator. In Fig. 49 the open- 

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

 central part being stopped out, thus giving rise to dark-ground illumination ($ 92). 



Obj. Obj. The front of the objective. 



composed 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 



