THE DIAPHRAGM. 169 



are usually included, many of which are essentially 

 necessary for the prosecution of microscopical pursuits 

 and for the perfect examination of most objects. 



The Diaphragm, fig. 99, is a circular plate with a 

 series of circular apertures cut in it. In fact, there 



FIG. 99. The Diaphragm. 



are two plates of brass, one being perforated with four 

 or five holes of different sizes, and arranged to revolve 

 upon another plate by a central pin or axis, the last 

 being also provided with a hole as large as the 

 largest in the diaphragm-plate, and corresponding in 

 situation to the axis of the compound body. The holes 



FIG. 100. Dr. Anthony's Stage Diaphragm. 



in the diaphragm-plate are centred and retained by 

 a bent spring that fits into the second plate, which 

 rubs against the edge of the diaphragm-plate and 

 catches in a notch. The blank space shuts off the light 

 from the mirror when condensed light is used. It is 

 impossible to dispense with the use of the diaphragm, 



