GeoDictrical Electron Optics 13 



be eliminated. Optical microscope objectives which are corrected 

 for spherical aberration and coma are called aplanats. The most 

 perfect specimens of the optical art, the apochromats, are aplan- 

 atic and chromatically corrected for three different spectral 

 colors. Electron microscope objectives, however, are entirely 

 uncorrected. Yet, these primitive lenses, which as regards opti- 

 cal perfection can be perhaps conipared to a dewdrop, have 

 beaten the marvels of the optical art by a factor of almost a 

 hundred. How has this been achieved? 



It is known that even the worst lens approaches perfection 

 if its aperture is reduced to almost nothing. The high quality 

 of present-day electron microscopes has been achieved by this 

 simple artifice of using objectives with pinhole apertures or their 

 equivalents. This is a line which light optics could never follow, 

 as two phenomena would soon set a limit to it : diffraction at the 

 aperture and insufficient light intensity. In electron optics, 

 however, it was highly successful for two reasons. The first is 

 the smallness of the diffraction effect, due to the extreme short- 

 ness of the de Broglie wavelength of the electrons, which will 

 be discussed in the next chapter. The second reason is that the 

 intensities which can be realized in electron optics are enormously 

 greater than those available in light optics. 



Intensity is defined in optics as the light flux per unit area 

 at right angles to the stream and unit solid angle, i.e., per cm- 

 and steradicui. It is known that the intensity divided by the 

 square of the refractive index of the medium is an invariant 

 along a beam of light. This means that if the light originates in 

 a medium of refractive index ;/o and the original intensity is /q, 

 the intensity / which can be produced in this beam in a medium 

 n cannot be more than 



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though it can be less if some light has been lost by absorption or 

 reflection. With the media available in light optics this means 

 that the intensitv can be increased at the best three to four times. 



