326 



COMPOUND AND ELECTRON MICROSCOPES 



Whether or not the cone of light is correct can be tested easily by- 

 observing the upper lens of the objective. It should be filled with an 

 even illumination. This can be observed if one removes the eyepiece 

 and looks down the draw tube of the microscope. This should be done 

 each time one starts to work with a microscope and each time one 

 changes objectives. 



Suppose that an objective of 1.25 N.A. is used with a condenser of 

 1.4 N.A. If a layer of air (index 1.00) is left between the top lens of the 

 condenser and the slide, the air layer will prevent the condenser from 

 delivering a cone of light greater than 1.00 N.A. By substituting a 



layer of immersion oil between the slide and 

 the top lens of the condenser the full numeri- 

 cal aperture of the condenser becomes available 

 and the full aperture of the objective will be 

 used. In practice the 1.40-N.A. condenser must 

 be diaphragmed, down to the numerical aper- 

 ture of the objective, in this case 1.25. 



Abbe Condenser 



For objectives of numerical aperture greater 

 than 0.25, the ordinary substage mirror is not 

 large enough to provide an illuminating cone of 

 light of sufficient angle to fill the aperture 

 of the objective. This objection is met by in- 

 troducing a substage condenser or an Abbe con- 

 denser between mirror and object slide (Fig. 

 VIII-10). Condensers are usually neither chro- 

 matically nor spherically corrected. Their function is to send a suffi- 

 ciently large cone of light through the object to fill the aperture of 

 the objective. 



These condensers may be obtained in various numerical apertures: 

 1.25 N.A., containing two lenses with top lens removable, or 1.40 N.A., 

 containing three lenses. The condenser mount fits into the substage 

 from below and is provided with an iris diaphragm. If an achromatic 

 condenser is obtained, it is corrected for two colors and spherically cor- 

 rected for two zones. It is used where it is necessary to have a sharp 

 image of the light source, free from color fringes, projected into the 

 plane of the object. The optical structure of the condenser is such 

 that only parallel rays of light are brought to a focus, in the plane of 

 the object under examination. To obtain this parallel beam of light, 

 a condenser must be interposed between the plane face of the substage 

 mirror and an artificial point source, which parallelizes the beam of light. 



Fig. VIII-10. Abbe 

 or substage condenser, 

 showing the relative size 

 of its 1.20 and 1.25 num- 

 erical aperture. 



