8 



PRACTICAL HISTOLOGY. 



fitted small diaphragms perforated by apertures of different sizes 

 (figs. 10, 12). The cylinder, with its diaphragm, is fitted into a 

 slot under the stage. These cylinder diaphragms should be so 

 arranged as to be easily changed. 



Perhaps the most convenient form of all is the iris diaphragm, 

 which can be adapted to any of the larger microscopes (fig. 14), 

 and in which any size of aperture desired is obtained by turning a 

 small milled head. The new form (fig. u), as made by English 



makers, Zeiss, and others, 

 and also by Leitz, is 

 an admirable substitute 

 for interchangeable dia- 

 phragms, as by it the 

 aperture can be readily 

 increased or diminished. 

 The smallest aperture is 

 about 0.5 mm., and the 

 largest equal to the full 

 aperture of the con- 

 densing system. 



The management of 

 the diaphragm is most 

 important in order to 

 obtain distinct definition 

 of an object. The one 

 general rule but one 

 which is very frequently 

 neglected in practice by 

 the student is when 

 employing a low power 

 to use a large aperture, 

 and when employing a 

 high power, a small or 

 medium aperture of the 

 diaphragm. 



FIG. io. Zeiss's Large Jointed Stand, fitted with draw- When O116 wishes to 

 tube, rack and pinion for the coarse adjustment, a , , . -, , , 



double nose-piece, and cylindrical diaphragm. Observe a brightly col- 



oured object with a 



homogeneous immersion lens the object lying in the tissues, such 

 as stained fungi or mitotic figures then remove the diaphragm and 

 allow a flood of light to reach the sub-stage condenser, which is 

 absolutely necessary in this case. 



If, however, it be desired to see certain peculiarities of structure 

 as in bacteria on a black background, then use a central stop- 

 diaphragm, i.e., one with a central stop (fig. 12), always with an 



