SIZE, SHAPE, AND HYDRATION OF VIRUSES S7 



revolution, into a beam of monochromatic light. The centrifngal 

 action causes a virus boundary to develop, with solvent on the 

 inside and the outward moving virus on the outside. For viruses, 

 this soon becomes very sharp, and, at the boundary, very rapid 

 change of refractive index occurs. The position of this boundary 

 can be made visible by Topler's schlieren method, and the 

 optical system accordingly includes a lens, which focuses an 

 image of the cell onto a photographic plate, and an edge across 

 the lens. The effect of a changing refractive index is to send 

 behind the edge a part of the cell image and, accordingly, to 

 have a part of the image missing on the photographic plate. 

 This can be modified by using an oblique bar and a cylindrical 

 lens to give an up and down deflection of the trace at the bound- 

 ary. Such patterns are very familiar in the literature of macro- 

 molecules today. One such is shown in Fig. 2.4, where the virus 

 being spun is southern bean mosaic. The virus preparation can 

 be seen to be not quite homogenous, and a filter placed across 

 the light source produces a definite absorption at the boundary 

 of the impui-ity. The main sedimentation occurs at the expected 

 rate. This picture was taken by Mr. D. Caspar in the author's 

 laboratory. 



The speed of the ultracentrifuge is accurately controlled by 

 a relay system, and pictures can be taken at known intervals 

 so that the sedimentation constant can be measured directly. 



A very beautiful development of the ultracentrifuge has 

 recently been made by Beams, Ross, and Dillon (1951). If a 

 strong magnetic field has both a radial and an axial gradient, 

 then a rotor of ferromagnetic material will seek the region of 

 highest field and, having found it, will remain suspended there 

 as long as the magnetic field is present. In Beams' device, the 

 rotor is brought up to speed, released of all material contact, 

 and left to spin in an evacuated chamber with magnetic support. 

 The rotor loses one revolution per second per day, and so the 

 motion is essentially uniform. Two years would be required for 

 the rotor to come to rest if left alone. This method is suitable 

 for equilibrium studies in which the sedimentation drift is 

 balanced by the tendency to diffuse. This may be of more 



