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. 
