22 THE PHYSICS OF VIRUSES 



and of greatly variable lengths. Similar results, but in lower 

 concentration, were observed for potato virus Y, potato para- 

 crinkle, henbane mosaic, tobacco etch, and cabbage blackring. 

 For cucumber mosaic virus, they observed variable length rods 

 150 A wide in about one-thousandth the concentration of 

 tobacco mosaic virus. Spherical viruses were observed for 

 tobacco ringspot (260 A diameter) and Rothamsted tobacco 

 necrosis, which showed two sizes of particle 180 A and 380 A in 

 diameter. 



The development of the growth of psittacosis virus is chorio- 

 allantoic membranes has been followed by Heinmets and 

 Golub (1948). Definite development could be seen. The size at 

 72 hr was 3,600 A diameter. 



Valuable as is this kind of work, it suffers from the disadvan- 

 tage that an enormous amount of understanding of cellular 

 constituents is required before confident assertions can be made. 

 After all, the electron microscope permits observation to nearly 

 100 times the fineness of detail of the optical microscope, and 

 this means, then, thousands of times in area. All of this is 

 largely unexplored and undescribed. Besides this complication, 

 the preparative techniques of electron microscopy introduce 

 different selections of special cellular constituents than optical 

 microscopy, and, until all of these features are sorted out, the 

 electron microscope must be used with care on tissue slices. 



Ultrafiltration 



This method has the outstanding advantage of universal 

 applicability and does not need either highly purified prepa- 

 rations or specially high concentration. The method was started 

 by Elford (1931) who used graded collodion membranes pre- 

 pared by depositing collodion layers of difTerent composition. 

 In this way a graduation in pore size of filter can be obtained. 

 To determine the pore diameter, a combined technique of 

 viscous flow and fluid absorption is used. The filter is considered 

 to be idealized as shown in Fig. 2.3. It is assumed that there are 

 n cylindrical pores of radius r and length /. Then, in terms of 

 Poiseuille's law of viscous flow, the volume, V, of liquid of 



