DARK GROUND ILLUMINATION 



181 



the lever while the object is actually under examina- 

 tion. 



The diagram Fig. 54 shows the course of the rays 



Fig. 54 



DIAGRAM OF DAEK GROUND ILLUMINATION BY CENTRAL STOP 



through Watson's universal condenser when a central 

 stop is placed beneath it. As only the marginal rays are 

 used to illuminate the object, and they do not enter the 

 objective, the image is formed by rays diffused from the 

 object only, and exposure is rather long unless a powerful 

 illuminant is used. The condenser must also be of 

 considerably greater aperture than the objective ; for a 

 J", for instance, it is almost essential to use an oil-immer- 

 sion condenser, and its entire aperture must be illuminated 

 by means of an auxiliary condenser. It is advantageous 

 to have at hand some means of decreasing the N.A. of 

 the objective ; an internal conical stop is now supplied 

 for use with TV" oil-immersion objectives, but it is usual 

 with lower powers to employ a Davis iris diaphragm 



Fig. 55 



DAVIS 5 DIAPHRAGM 



(Fig. 55) between the objective and tube of the micro- 

 scope. 



