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



125a. Reducing diaphragm for oil immersion objectives. It necessarily 

 follows that if the condenser transmits light between the apertures of i.oo and 

 1.40 N.A., that the objectives used must have an aperture less than i.oo N.A. or 

 some of the light from the condenser would pass into the objective and be trans- 

 mitted through the microscope to the eye, and that would destroy the dark- field 

 effect. As oil immersion objectives are used in dark-field work, and practically all 

 of them have an aperture ranging from the axis to 1.20, 1.30 and 1.40 N.A. at the 

 outer limit, some means must be used to bring the outer limit of the aperture of 

 the objective below the outer limit of the aperture of the condenser. To accom- 



FIG. 48. PARABOLOID CONDENSER FOR DARK-FIELD ILLUMINATION. 



Axis The principal optic axis of the condenser and of the microscope. 



C-r Face- view of the top of the condenser showing the centering ring, the spot 

 of white ink in the middle and the grains of starch scattered over the face of the 

 condenser to aid in focusing high powers on the centering ring. 



C-S, C-S Sectional and face views of the central stop to cut out all light below 

 an aperture of i.oo N.A. 



Cover Glass The glass cover over the object. For dry objectives it must be of 

 the right thickness for the objective (461). For homogeneous immersion objec- 

 tives the cover glass must be less than the working distance ( 76). 



Hi, Hi Homogeneous liquid between the condenser and the underface of the 

 glass slide, and between the upper face of the cover glass and the objective. 



NA i.oo to NA 1.40. The numerical aperture of the hollow cone of light 

 focused upon the object. This hollow cone is t represented by a glass angle of 67 

 to 41 degrees as indicated on the left of the condenser and on the right above the 

 condenser. 



Objective The front lens of the homogeneous immersion objective. The light 

 rays deflected by the object are indicated by white lines below and through the 

 objective, and by broken lines above. 



MM The mounting medium. It may be of any refractive index from air to 

 Canada balsam. 



