29 : 8/ Microscopy 



553 



worse than the theoretical limit, one can still obtain separate images 

 from two points whose distance apart b is 



b = 10A 



This is about the actual limit of resolution in the better electron micro- 

 scopes. 



Electron lenses are necessary to focus the electron beam if one wishes 

 to obtain magnification. As discussed in the previous paragraph, when 

 the potential changes, the electron wavelength changes, but its frequency 

 remains approximately constant. Accordingly, one may treat V as 



Lamp 

 Condenser Lens 



Object 

 Objective Lens 



Real Image 

 Eyepiece Lens 



Eye 



Cathode and Anode 

 Condenser Lens 



Object 

 Objective Lens 



Real Image 

 Projector Lens 



I 2 Real Image 

 Screen 



Light 

 Microscope 



Electron 

 Microscope 



Figure 10. A comparison of. the light microscope and the 

 electron microscope. After G. P. Svvanson, The Cell (Engle- 

 wood Cliffs, N.J.: Prentice-Hall, Inc., 1960). 



analogous to n, the index of refraction for light waves. Properly shaped 

 electrostatic fields act on electrons just as glass lenses of the same shape 

 act on photons. This type of electron lens is called an electrostatic lens. 



Another type of electron lens is the so-called "electromagnetic type." 

 Electromagnetic theory shows that electrons moving in a magnetic field 

 effectively experience an increase in potential. Electromagnetic coils 

 around the beam can form lenses just as can the electrostatic ones. The 

 detailed theory is somewhat more complex and will not be pursued here. 



Whether electromagnetic or electrostatic lenses are used depends on 

 practical engineering details. In either case, the parts of an electron 

 microscope are analogous to those of a light microscope. This analogy 

 is presented in Figure 10. The electrons are emitted from a heated 



