THE PHYSICAL PROPERTIES OF INFECTIVE PARTICLES 247 



indication of heretogeneity. However, slight deviations were often undetect- 

 able, and homogeneity was ascribed to preparations which are now known 

 to have contained populations of macromolecules of varying size. This can 

 be attributed, no doubt, to limitations in the optical methods then used. 

 Boundaries which did not maintain their symmetry during a diffusion experi- 

 ment, as was observed in early experiments with tobacco mosaic virus 

 (Neurath and Saum, 1938), were an indication that the diffusion coefficient 

 varied with concentration. For such systems Equation (11) is not 

 applicable, and a more elaborate treatment of the data is neces- 

 sary. 



Despite improvements in the quality of the lenses employed in the schlieren 

 optical systems, further progress was limited for both experimental and 

 theoretical reasons. Therefore developments in the adaptation of intei- 

 ferometric optical techniques were greeted with enthusiasm, and now two 

 different optical methods have been adopted widely. The first is the so-called 

 Gouy interference method (Kegeles and Gosting, 1947) and is the simpler 

 of the two. Instead of a smooth curve of refractive index gradient versus 

 distance, the Gouy system gives a series of horizontal, closely spaced inter- 

 ference fringes from which the concentration gradient curve can be con- 

 structed with great precision. The coordinates of the gradient curve are 

 evaluated, in effect, from the positions of the fringes which arise from the 

 interference of pairs of light rays which in traversing the boundary are 

 deviated exactly the same amount and arrive at the same position at the 

 focal plane of the lens system. Since one of each pair of rays experiences a 

 different optical path from the other, the rays may be in or out of phase, 

 giving constructive or destructive interference. This depends on both the 

 refractive index at the two levels within the boundary and the geometrical 

 distances traversed by the hght. Early in the diffusion experiment the lines 

 are spaced widely with the lowermost fringe, which corresponds to the maxi- 

 mum concentration gradient, being displaced a large distance from the 

 undeviated hght. As diffusion progresses, the fringe spacing diminishes 

 although their number remains constant. From appropriate tables and 

 measurement of the fringe positions, the diffusion coefficient can be calcu- 

 lated. The constancy of the values from different fringes is an indication of 

 the Gaussian nature of the boundary and therefore a valuable criterion of 

 the homogeneity of the diffusing substance. Owing to the fact that the 

 optical system does not contain a lens which images the cell, the location of 

 the boundary within the cell cannot be recorded by this method; the 

 Gouy system is, therefore, not useful for studies of moving boundaries such 

 as are met in electrophoresis or sedimentation velocity experiments. For 

 shapes of boundaries, however, this method is misurpassed. 



The second method, known as the Rayleigh interference method, gives a 



