ILLUMINATION OF OBJECTS; DARK FIELD 



39 



illuminator in which, Fig. 15, a lens is combined with a parabo- 

 loid to bring the rays to a proper focus. 



FIG. 14. Types of Dark-field Illuminators. 

 A. Nachet et Fils. B. Reichert. 



C. Bausch & Lomb. D. E. Leitz. 



The Beck illuminator is unique in that it permits adjustment 

 for different thicknesses of object slides, an impossiblity with 

 other forms of paraboloid illuminators. / 



This adjustment for slide thickness is *> ^*^..,.^ * * 



accomplished by changing the distance 

 between the focusing lens L and the para- 

 boloid P. As seen in the diagram, the 

 illuminator consists of two parts, the 

 paraboloid mounting screwing into that 

 which holds the lens; therefore raising or 

 lowering the paraboloid will displace the 

 focal point / and bring about an accom- 

 modation for different thicknesses of slides. 



In practice it is rarely possible to have such accurate grinding 

 that all the rays are properly deflected and none enter the objec- 



FIG. 15. Beck Adjustable 

 Dark-ground Illuminator. 



