DARK GROUND ILLUMINATION 



185 



Fig. 59 



COURSE OF RAYS IN LEITZ' DARK GROUND ILLUMINATOR 



In order that the rays, which, as indicated in the figure, 

 are very oblique, shall not suffer total reflection at the 

 under surface of the slip, the latter is always oiled to the 

 condenser. The light consequently passes through to 

 the object, and when a dry objective is used, any rays 

 that may reach the cover-glass are totally reflected, and 

 the only rays utilised by the objective are such as are 

 deflected by the particles of the preparation, and emitted 

 at an angle within the aperture of the objective. On the 

 other hand, when the objective is oiled to the cover-glass 

 these oblique rays can pass direct to it, and be more or 

 less utilised according as its aperture is greater than 

 N.A. 1. This would prevent the formation of a dark 

 field, so a conical stop restricting the aperture to 1-0 is 

 screwed into the objective mount with its apex close to 

 the back lens. 



The Ultramicroscope. Small particles in a preparation 

 are made visible in proportion to the contrast with which 

 they are displayed against the background, and the 



