Principles of Microscopy 17 



aperture of 0.9. The photograph was then enlarged ten times to render 

 obvious in reproduction details on the absolute limit of resolution both 

 of the lens and the observer's eye. The minute speck of detached silver 

 ( 1 ) thus appeared on visual observation as a just visible particle. Notice 

 also that the leg-shaped projection to the right of this particle has a tiny 

 bump in the sole of the foot. At (2) there is a narrow, but relatively 

 deep, tear in the metal. 



Now turn to the lower figure (Fig. 17). The only difference in the 

 setup for taking this photograph was that the iris of the substage con- 

 denser was shut down until the lens was operating at N.A. 0.2. Notice 

 first that the fine particle (1) and the bump in the sole of the footlike 

 projection have vanished. They are, like the deep tear at (2), beyond 

 the limit of resolution of a lens of N.A. 0.2, and the fact that the objective 

 in question has N.A. 0.95 engraved on the barrel has nothing to do with 

 its performance. Not only, however, has closing the substage iris caused 

 fine detail to vanish or become distorted, but it has introduced many de- 

 tails that are not there. Notice particularly the appearance of nonexistent 

 "vacuoles" in the solid metal. Those who believe, on the basis of the 

 entirely false analogy of a camera lens, that "closing the iris brings out 

 the detail" are likely to come up with some staggering inaccuracies of 

 observation. 



The top photograph (Fig. 15) shows the result of opening the sub- 

 stage iris too wide. The substage condenser used for these photographs 

 had a maximum aperture, since it was oiled to the slide (Fig. 11), of 

 N.A. 1.4. The cone of light entering the objective was therefore wider 

 than the objective could handle. In consequence of this there was much 

 internal reflection from lens barrel and microscope tube. These internal 

 reflections produce "glare," which makes it impossible to see, and very 

 difficult to photograph, fine detail. In sum, the critical adjustment of the 

 substage iris is one of the most important factors in the use of the micro- 

 scope. The method of making these adjustments in practice is described in 

 the next chapter. The present discussion has been for the sole purpose 

 of emphasizing that the substage iris controls resolution, not light in- 

 tensity. Before passing to sources of illumination, it is necessary to draw 

 attention to another criterion of the substage condenser— working dis- 

 tance and focal length. 



Figures 9 through 14 show clearly that the substage condenser focuses 

 the light on the object. Actually, as will be explained in the next chapter, 

 it focuses either an image of the light source or of an iris in front of the 

 light source. The size of this image is naturally dependent both on the 

 focal length of the condenser and on the distance of the light source 

 from the condenser. A very high N.A. obviously necessitates a short 

 working distance, which results in a short focal length and a very small 



