16 



The Microscope 



Figs. 15, 16, and 17. The effect of numerical aperture, as controlled by the substage 

 iris, on resolution and glare. Fig. 16 (B) is taken with a X45 apochromatic objective 

 working at N.A. 0.93 at a magnification of 1,500. The object shown is the torn edge 

 of a thin sheet of silver foil and the speck at 1 or slit at 2 lies at the absolute limit of 

 resolution possible at this N.A. Closing the substage iris to produce an N.A. of 0.2 

 [Fig. 17 (C)] not only causes the speck and slit to vanish but inserts a lot of non- 

 existent vacuoles into the picture. Opening the iris to give an emergent cone of N.A. 1.3 

 into a lens working at N.A. 0.93 [Fig. 15 (A)] produces glare that masks the image. 



