THE PHYSICAL PROPERTIES OF INFECTIVE PARTICLES 289 



this aberration alone there is a certain minimum resolvable distance that can 

 be written: 



d,^^=Cd^ (38) 



where C is a constant depending upon the focal power of the lens and upon 

 the variation of the magnetic field along its axis, and 6 is the aperture angle 

 as used in Equation 37. 



It is evident that a compromise must be effected in the design and use of 

 electron lenses, inasmuch as changing the value of 6 affects the value of the 

 minimum resolvable distance in opposite senses when the effects of both 

 diffraction (Equation 37) and of spherical aberration (Equation 38) are con- 

 sidered. It can be shown that the sum of the two effects is minimized when 

 they are made equal to each other, i.e., when d^iff = d^^^^. It turns out that 

 6 has a value of only about 1 degree imder these conditions. Hence the 

 resolving power of an electron microscope is only about 1/100 as great as it 

 would be if lenses could be made having the numerical apertures of objective 

 lenses commonly met with in light microscopes. It is generally agreed that 

 an electron microscope can have a minimum resolvable distance as small as 

 6 A when it is used to photograph ideal objects: small, discrete particles of 

 high electron opacity. 



in. Effects of Specimen Thickness on Resolution and Contrast. In practice 

 it is unusual for a specimen to have ideal properties for electron imagery, 

 and in the case of viruses, at any rate, the effective resolving power of the 

 electron microscope is notably inferior to the 6A mentioned above. There 

 are two important factors affecting this additional limitation upon resolving 

 power: thickness of the specimen, and contrast between specimen object and 

 the background. When electrons penetrate an ordinary specimen they are 

 scattered, most of them elastically but some inelastically. The inelastically 

 scattered electrons wall suffer a decrease in velocity, and hence wiU not be 

 focused in the same jDlane as the main electron beam. The effect is to degrade 

 the image sharpness in a manner analogous to the effects of chromatic 

 aberration in a glass lens system when white light is employed. The conse- 

 quences of chromatic aberration in electron lenses are serious when the 

 specimen thickness is greater than a few hundred Angstrom units. 



The source of contrast in an electron microscope is different from that in 

 a light microscope. In the latter the contrast is due to differential opacity of 

 the specimen objects; light of certain wavelengths is actually absorbed here 

 and there in the specimen. But all the light leaving the specimen is brought 

 to a focus where the image is observed. In phase-contrast and interference 

 microscopy the specimen need not absorb light to exhibit contrast, but rather 

 it need only have variations of thickness and/or refractive index from point 

 to point. But here, too, all the hght leaving the object plane is imaged by the 

 objective lens. In an electron microscope (if we neglect inelastic scattering) 



