18 The Microscope 



image. This image is worthless if it does not fill the field of view of the 

 objective, so there are objections as well as advantages to achromatic 

 N.A. 1.4 condensers. 



The ordinary Abbe condenser, with a maximum working N.A. of about 

 0.9, is perfectly adequate for most work The relatively long focal length 

 gives a working distance sufficient to permit focusing through even the 

 thickest slide and produces an image of an adequate lamp sufficiently 

 large to fill the field of a XlO (16-mm) objective. The resolution of an 

 oil-immersion objective of N.A. 1.2, working at N.A. 0.9 with an Abbe 

 condenser, is good enough for all student uses and for routine laboratory 

 work. 



The N.A. 1.4 achromatic condenser is only required when the highest 

 possible color correction is sought with apochromatic objectives or when 

 oil-immersion lenses are to be used at numerical apertures greater than 1. 

 In this case, of course, the condenser, as well as the objective, will have 

 to be oiled to the slide. The image of most light sources produced by 

 these high-power condensers is too small to fill the field of even medium- 

 power objectives, and there are only two ways to get around this. Either 

 the light must be brought closer or the focal length of the condenser 

 increased. The former is what in effect happens either with a built-in 

 light source or with the American Optical illuminator described in the 

 next section. The focal length of the condenser can be increased in two 

 ways. The simplest, but least satisfactory, is to insert a low-power lens 

 into the system immediately under the condenser. Many microscopes are 

 fitted with either a swinging or sliding lens for this purpose. Unfortunately, 

 these lenses not only expand the beam but also throw it out of focus, so 

 that the corrections and resolution of the low-power objectives are seri- 

 ously affected. The proper function of these swinging or sliding lenses 

 is to permit a field to be searched with low power to locate an object 

 for examination under high power. 



The proper setup for critical microscopy with low- or medium-power 

 lenses is shown in Fig. 14. Almost all achromatic condensers have the 

 top lens mounted in a knurled ring. This top lens can thus be easily un- 

 screwed and removed without seriously affecting the corrections of the 

 remaining lenses of the system. The lower-power condenser so produced 

 usually has an N.A. of about 0.5 and produces an image of the light source 

 which will fill the field of a X5 (32-mm) objective. A few achromatic 

 condensers are made so that the two top lenses of the system can be 

 removed, but this is only necessary for low-power photomicrography, for 

 which purpose it is usually better to buy a specially built condenser. 



