Principles of Microscopy 15 



achromatic objectives. An "apochromatic" condenser is not, of course, 

 a feasible proposition since there is no possibility of inserting into the 

 system a supplementary compensating ocular to draw the widely split 

 rays together again. 



The critical criteria of substage condensers are maximum N.A., work- 

 ing distance, and focal length. The term "maximum" N.A. is used ad- 

 visedly, because every substage condenser is furnished with an iris 

 diaphragm the only function of which is to control the N.A. It is a wide- 

 spread delusion, ruinous to good microscopy, that this iris may be used 



SMALL IMAGE OF LIGHT SOURCE 



IRIS DIAPHRAGM 



NA 02 



LARGE IMAGE OF LIGHT SOURCE 



12 



13 



Figs. 12, 13, and 14. Diagrams to show the effect of the substage iris on numerical 

 aperture and on the size of the image of the light source. Fig. 12 shows an Abbe 

 condenser, with the iris wide open, casting a small image of the light source with a 

 light cone of 83° and a consequent N.A. of 0.7. Fig. 13 shows the same condenser 

 with the iris partially closed casting the same size image of the light source but with 

 an angular aperture of 28° so that the N.A. is now 0.2. Fig. 14 shows an achromatic 

 condenser with the top lens removed casting a large image of the light source so that 

 a low-power objective can be used. 



to control the intensity of illumination. Reference to Figs. 12 and 13 

 should make this point clear. Figure 12 shows an Abbe condenser, with 

 the iris almost fully open, throwing a cone of light with an 83° angle. 

 Any lens placed above this will therefore be functioning, if it is con- 

 structed to do so, at an aperture of N.A. 0.7. Figure 13 shows the same 

 condenser with the iris diaphragm partially closed. The emergent cone 

 of light now has an angle of 28°, and any objective placed above it can- 

 not operate at more than N.A. 0.2, no matter what figure is engraved on 

 its barrel. It is true that decreasing the N.A. has the effect of diminishing 

 the light intensity but it is at the cost both of losing fine detail and, in 

 extreme cases, of introducing detail that is not there. This is well seen 

 in Figs. 15, 16, and 17. Figure 16 is a photograph of the jagged torn 

 edge of an extremely thin layer of silver foil. This is an excellent test 

 object because there is no question of what it ought to look like. It ought 

 to appear as a clean silhouette with crisp edges. This photograph was 

 taken with an apochromatic X45 (4-mm) objective operating at an 



