6 • The Electron Microscope 



ergs per degree. Expressed in electron-volt units, or shorter in 



. . T 

 "volts," this is -- — - volts. For oxide-coated cathodes, this is 



5800 



about 0.17 volts, and for tungsten cathodes 0.43 volts. Calling 

 this average initial value ^o, it is convenient to define the refrac- 

 tive index as follows 



n — =z — (1) 



In this expression, the arbitrary constant factor has been dis- 

 posed of in such a way as to make the root mean square refractive 

 index unity at the cathode. This is only slightly different from 

 the average refractive index. 



Equation (1) discloses an important quantitative difference 

 between light and electron optics. Whereas the refractive index 

 of optical media varies only in the limits 1-2, in an electronic 

 device in which electrons emitted by a barium cathode are ulti- 

 mately accelerated to 100,000 volts the refractive index varies in 

 the limits 1-1,000. This is the main intrinsic advantage of 

 geometrical electron optics, to which we shall return several 

 times. 



We can now immediately explain the lens effect of an axially 

 symmetrical electric field, of which figure 1 shows an example. 

 The equipotential surfaces are figures of revolution. They inter- 

 sect the axis at right angles, and in the neighborhood of their 

 vertex they are approximately spherical. If we imagine the 

 space between these surfaces filled with a medium of a refrac- 

 tive index proportional to \/ <^, we obtain a succession of lenses, 

 somewhat distorted outside, but of the correct shape near the 

 axis. As a combination of aligned lenses is again a lens, the 

 imaging effect of the electric field is immediately evident. In 

 the figure, increasing refractive index is indicated by shading of 

 increasing density. 



The explanation of the magnetic type of lens is somewhat 

 more complicated. The force exerted by a magnetic field on a 



