56 



THE PHYSICS OF VIRUSES 



still smaller crystalline preparations, Bernal, Fankuchen, and 

 Riley (1938) found a spherical particle diameter of 270 A for 

 bushy stunt virus. 



If in these crystals each virus could be identically oriented, it 

 would be possible to find out some information about the internal 

 atomic spacing. For spherical viruses this has not been done, 

 but for tobacco mosaic virus, the long rod shape does frequently 

 cause rather accurate mutual orientation. Using such prepara- 

 tions, Bernal and Fankuchen (1941) found that there is an 



Fig. 2.10. Unit cell of tobacco mosaic virus as determined by Bernal and 

 Fankuchen (1941). 



internal structure with a hexagonal lattice. To make their data 

 fit this lattice, they found it necessary to use some fractional 

 values of tt in the Bragg formula, where n is supposedly integral. 

 Bernal and Fankuchen i:)oint out that in a virus there is nowhere 

 near an infinite atomic array. Hence the atomic planes do not 

 select precisely integral values for scattering. If this interpreta- 

 tion of their data is right, the unit cell they find is as shown in 

 Fig. 2.10. The hexagon side is 87 A long, and the cell depth is 

 68 A. It is very gratifying that the precision electron microscopy 

 applied by Williams (1952) to this virus also shows a hexagonal 

 cross section of the same size. Thus Bernal and Fankuchen's unit 

 cell has one cross section which is that of the virus itself. 



The same method has been applied to tomato aucuba mosaic 

 virus, enation mosaic virus, and cucumber viruses 3 and 4, all 

 of which are related to tobacco mosaic virus. The same width 



