ILLUMINATION OF OBJECTS; ILLUMINATING DEVICES 43 



illuminator in which, Fig. 16, a lens is combined with a paraboloid 

 to bring the rays to a proper focus. 



The Beck illuminator is unique in that it permits adjustment 

 for different thicknesses of object slides, an impossibility with 

 other forms of paraboloid illuminators. ^ 



This adjustment for slide thickness is 

 accomplished by changing the distance 

 between the focusing lens L and the pa- 

 raboloid 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- FlG - l6 - Beck Adjustable 

 modation for different thicknesses of slides. Dark -g round illuminator. 



In practice it is rarely possible to have such accurate grinding 

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

 tive. Only those rays included in a low numerical aperture are 

 available. Hence the employment of an objective of high nu- 

 merical aperture and very short working distance yields a field 

 which is never dark. Since practically all high power immersion 

 objectives are made with as high numerical apertures as possible, 

 it is absolutely essential that some means be used to reduce 

 their numerical aperture below i, if they are to be employed in 

 dark-ground studies. This is accomplished by introducing into 

 the objective mount some form of diaphragm : or specially con- 

 structed objectives of N.A. less than i may be purchased. 

 Diaphragms for use with objectives in dark-ground studies are 

 generally supplied by the manufacturers of reflecting condensers 

 for introduction into the special objectives to be used. These 

 funnel-like diaphragms are not interchangeable and can be 

 employed only for Jthe objective for which they are designed. 

 Figs. 17, 1 8 and 19 show three different types and forms of dia- 

 phragms employed for this purpose. In the case of Fig. 17 the 

 lens mounting is unscrewed just back of the back lens combina- 

 tion and the funnel diaphragm, provided with male thread, is 

 screwed into the opening tapped into the upper half of the objec- 



