40 



ELEMENTARY CHEMICAL MICROSCOPY 



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

 available. Hence the employment of an objective of high 

 numerical 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-field studies. This is accomplished by 

 introducing into the objective mount some form of diaphragm: 

 or specially constructed objectives of N.A. less than i may be 



Fig. i 6. 



Fig. i 8. 



Fig. 17. 



Methods of Reducing Numerical Aperture of Objectives for Dark-field Studies. 

 (D, D, D, Removable Diaphragms.) 



purchased. Diaphragms for use with objectives in dark-field 

 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 the objectives for which they are designed. 

 Figs. 16, 17 and 18 show three different types and forms of dia- 

 phragms employed for this purpose. In the case of Fig. 16 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- 

 tive mounting. In the case of Fig. 17, the objective is also 

 unscrewed just above the back lens combination, but in this 

 case the diaphragm is merely dropped into the hole in the lower 



