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. 



t 



^v& 



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. 



