224 H. K. SCHACHMAN AND R. C. WILLIAMS 



a. Particles of Any Shape 278 



b. Spherical Particles 278 



3. X-ray Diffraction 279 



a. The Simple Lattice 279 



b. The Compouiid Lattice 281 



c. Calculation of X-ray Litensities 282 



d. Fourier Summations 283 



e. Patterson Vector Maps 284 



f. Heavy Atom Replacement 284 



g. Diffraction Patterns of Oriented Fibers 286 



4. Electron IVIicroscopy 287 



a, Priaciples of the Formation of an Electron Image 287 



i. Resolving Power of an Optical System 287 



ii. Resolving Power of an Electron Microscope 288 



iii. Effects of Specimen Thickness on Resolution and Contrast 289 



b. Techniques of Electron Microscopy 291 



i. Virus Suspensions, Morphology 291 



ii. Virus Suspensions, Quantitative 293 



iii. Sections of Infected Cells 294 



5. Radiation Inactivation 294 



a. The Effects of High-Energy Radiation 294 



b. The Target Theory 295 



III. Application of Physical Methods to Virus Studies 297 



A. Homogeneity and Purity as Related to Virus Identification 297 



B. Identification of Physical Particles as Infective Agents 299 



1 . Electron Microscopy 300 



2. Sedimentation 301 



C. Physical Properties of Well-Studied Viruses 303 



1. Tobacco Mosaic Virus 303 



a. Introduction 303 



b. Molecular Weight 304 



c. Homogeneity and Length 308 



d. Diameter and Cross -Sectional Shape ; Surface Structure 311 



e. Internal Structure 313 



2. Tomato Bushy Stunt Virus 316 



3. Turnip Yellow Mosaic Virus 318 



4. Poliovirus 321 



References 323 



I. Introduction 



When it became apparent about twenty-five years ago that the agents of 



virus diseases were submicroscopic particles which could be purified and 

 studied by methods of colloid chemistry, interest was initiated in the deter- 

 mination of their physical properties. The subsequent years have seen the 



development of many new methods for the study of colloidal systems, and 

 as these have appeared they have, without exception, been applied to the 

 examination of virus particles. Some of the methods have proved to be 

 particularly suitable to the study of viruses; these are the ones that will be 



