(■//. //.] 



I TGHTING AND FOCUSING. 



47 



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

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

 tive, the field will appear dark i Fig. 51 \. If now the object is com 



°>c-Ui 



50. 



Si- 



48. 49- 



Figs. 48 51. Sectional views of the Abbe Illuminator of 1.20 N. ■ I. snowing 

 various methods of illumination [\ 84), i^k- I s . axial light with parallel rays 

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

 ground illumination with a central slop 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 <i sex 

 ondary axis of tin- illuminator, and is therefore oblique to the p> iiit ipal a x is. 



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

 phrag m is placed between the minor and the illuminator . In Fig . /o the opening 

 is eccentric for oblique light, and in Fig. 51 the opening is a narrow ring, the 

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

 [188). 



Ob/. 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 pari 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 

 jeet will appear like a self-luminous one on a dark hack ground. This 



