322 



COMPOUND AND ELECTRON MICROSCOPES 





?^^ 



-^/ Cover glass 



'<&&&&&&*■ 



Oil 



Slide 



Fig. VIII-7. Typical section of an immersion objective, 2 mm, N.A. 1.25. The 

 object is embedded in an oil having same index of refraction as lens and cover glass. 

 The first lens is hemispherical; the object is relatively immersed in the lens at one of 

 the aplanatic points, thus forming a virtual image point at 0' . The other aplanatic 

 point is at the center of curvature of the lower face of the meniscus lens M; hence 

 no refraction takes place at this lower surface. 0' is also the aplanatic point for 

 the upper surface of M so that this lens produces a perfect point image of 0' at 0". 

 The diverging beam coming from the virtual focus 0" is brought to a focus by 

 the doublets D\ and D%. 



BG R 



B + G + R 



Apochromatic 

 Fig. VIII-8. 



