SIZE, SHAPE, AND HYDRATION OF VIRUSES 



55 



populated planes, d, and the angle of scattering, 6, by the Bragg 

 relation nX = 2rf sin 6, where n is an integer. 



An analysis of such crystals of tobacco necrosis virus was 

 made by Crowfoot and Schmidt (1945). The crystals used had 

 been grown for more than a year, and the largest was 2 X 1 X 

 0.5 mm. Both still and oscillation pictures were taken, with the 



Fig. '2. 9. Electron micrograph of a small crystal of tobacco necrosis virus, 

 taken by Wyckoff . Reprinted from Electron Microscopy by R. W. G. Wyckoff. 

 Copyright 1949, Interscience Publishers. New York, London. 



result that the basic pattern corresponds to the packing in con- 

 tact of a set of spheres of diameter 160 A. It is remarkable that 

 over 500 hr of X-ray exposure could be given this crystal without 

 any apparent change in its crystalline structure. This has an 

 important bearing on other results involving radiation sensi- 

 tivity. The great accuracy of more usual crystal analysis by 

 X-rays is not present here since the crystalline perfection of 

 chemical crystals is not matched by the virus crystals. With 



