ILLUMINATION OF OBJECTS; ILLUMINATING DEVICES 41 



ence. Special dark-ground illuminators are marked by the 

 manufacturers with the thickness of object slide for which they 

 are designed. 



The use of an Abbe condenser with dark-ground stop as a 

 substitute for special dark-ground illuminators is not to be 

 recommended since the obliquity of the rays is seldom sufficient 

 to prevent some light from entering the objective. The results 

 usually obtained are likely to be poor and unsatisfactory. 



Dark-ground Illuminators are condensers of such construction 

 that very oblique light rays are caused to converge, usually by 

 reflection, and to so strike the lower surface of a cover glass 

 placed over the preparation to be studied as to be totally re- 

 flected. To prevent axial light from passing through the illu- 

 minator an opaque stop is placed in the optic axis of the device. 



Cover Glass 



FIG. 14. Path of Rays in Reflecting Condenser for Dark-ground Illumination. 



The field is therefore black or nearly so, save for a slight halo at 

 its edges, while the objects appear bright or brilliantly colored 

 upon a dark background. 



In Fig. 14 a simple paraboloid reflecting illuminator is shown 

 diagrammatically in section, with the directions of the light 

 rays so exaggerated as to make clearer the reason the field of 

 view is dark. 



