THE PHYSICAL PROPERTIES OF INFECTIVE PARTICLES 287 



one turn of tlie helix; hence there is a meridional spot on the tenth layer 

 line. The appearance of diffraction patterns of helices are generally so char- 

 acteristic that there is no difficulty in recognizing such structures. 



4. Electron Microscopy 



a. Principles of the Formation of an Electron Image. The electron microscope 

 is a close optical analog of the ordinary light microscope in that it has a 

 source of radiation, a condenser lens, one or more objective and projector 

 lens, and it images the specimen by transmitted radiation. The source of 

 radiation is a hot filament from which electrons are emitted thermionically 

 in vacuo and subsequently accelerated through 50 to 100 kilovolts potential. 

 At these velocities the electrons have associated with them a wave of 

 length about 0.04 A; it is this extremely short wavelength that makes possible 

 the high resolving power of the electron microscope. Since moving electrons 

 are carriers of charge they are influenced in their motions by electric and 

 magnetic fields. A strong magnetic field that is coaxial with an electron 

 beam and that has both axial and radial variations will serve as a positive 

 lens and wiU focus a diverging electron beam to an image. 



The three principal lenses of an electron microscope are usually wire- 

 wound solenoids in the axis of which is a soft-iron core (the pole-piece) having 

 a central hole. The strength of such a lens is varied by changing the current 

 in the solenoid, and lenses with focal lengths as short as 0.2 cm. have been 

 produced. The function of the first lens, the condenser, is to image the source 

 of electrons near or in the plane of the specimen. The specimen objects, 

 such as viruses, are mounted upon a thin film, almost completely trans- 

 parent to the electrons, while the objects themselves must be nearly so. 

 The electrons passing through the specimen either emerge unaffected or are 

 scattered through small angles. Those that fall within a sufficiently narrow 

 cone, centered on the specimen, pass through the objective lens and are 

 imaged near the front focal plane of the projector lens. This lens casts a final, 

 real image of the specimen upon either a fluorescent screen for direct visual- 

 ization, or upon a photographic fllm where the exposure times are of the 

 order of a few seconds. When a change of focus is required it is accom- 

 plished by changing the strength of the objective lens, while changes of 

 magnification are the function of the projector lens. The sole purpose of 

 using an instrument as complicated as the electron microscope is to form 

 images whose resolution is greater than that found in the images formed by 

 a fight microscope (see Hall, 1953). 



i. Resolving Power of an Optical System. Whenever a field of particles is 

 imaged by transmitted light it is convenient to regard the process as one in 

 which the final image is an interference pattern created by the recombina- 

 tion of radiation that has been scattered with that which has passed 



VOL. I — 20 



