CII. //] 



LIGHTING AND FOCUSING 



57 



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 object will appear like a self-luminous 

 one on a dark back-ground. This form of illumination is most 



54 55 56 57 



FIGS. 54-57. Sectional vieivs of the Abbe Illuminator of i .20 N.A. show- 

 ing various methods of illumination ( \ 101). FIG. 54, axial light with parallel 

 rays. FIG. 55, oblique light. FIG. 56, axial light with converging beam. 

 FIG. 57, dark-ground illumination with a central stop diaphragm. 



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

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

 of th e m icroscope. 



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

 secondary axis of the illuminator, and is therefore obliqueio 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. 55 

 the opening is eccentric for oblique light, and in FIG. 57 the opening is a nar- 

 row ring, the central part being stopped out, thus giving rise to dark-ground 

 illumination ( /oj). 



Obj. Obj. The front of the objective. 



successful with low powers. It is well to make the illuminator 

 immersion for this experiment, (see 116). 



(A) With the Mirror. Remove all the diaphragms so that 



