74 DARK-GROUND ILLUMINATION WITH HIGH POWERS [Cn. II 



although the light passes abundantly into the condenser. It is practically all 

 reflected back into the condenser. If now some starch or flour is scattered over the 

 end of the condenser, the particles in optical contact will glow as if self-luminous 

 (fig. 



127. Slides and cover-glasses; Tube-length for dark-field 

 microscopy. As the object receives the greatest amount of light 



FIG. 50. 



DIAGRAMS TO SHOW THE REQUISITE ANGLE OF LIGHT IN GLASS TO 

 FILL THE HEMISPHERE ABOVE WITH LIGHT. 



As shown by the diagrams the numerical aperture (NA) of the light in the glass 

 in each case must equal the index of refraction (Ir) of the overlying medium. Any 

 light above this aperture is totally reflected back into the condenser (indicated in 

 black). 



a Glass with air above. The light in the glass must reach an angle of 41 

 (NA i.oo) in order to fill the overlying space with light. 



b Glass with water above. In this case the light in the glass must reach an 

 angle of 61 (NA 1.33) to fill the hemisphere above with light. 



c Glass with glycerine (Glyc.) above. Here the angle in the glass must reach 

 an angle of 75 15 (NA 1.47) to fill the overyling hemisphere with light. 



d Glass with homogeneous liquid (Horn, imr.) above. As there is no refrac- 

 tion in this case between the glass and the overlying medium, the angle must be 

 90 (NA 1.52) in order to fill the hemisphere above with light. 



