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DAVID R. BRIGGS 



region is thrown down at point P will be proportional to the rate of 

 change of the refractive index, n, with distance, x, within the cell. 

 This rate of change of the refractive index, dn/dx, will be proportional 

 to the rate of change of protein concentration, dc/dx, in the region of 

 the boundary, varying as we pass from pure buffer through the bound- 

 ary into uniform protein-containing solution, from zero up to a maxi- 

 mum and down to zero again. Thus, as the diaphragm {D) is raised, 

 the width of the shadow iX') will increase. If we plot the rate of 

 change of n with distance, dn/dx, in the cell, against the position, x, 

 in the cell, a graph of the form shown in Figure 6 will be obtained for 

 a symmetrical boundary. Such a figure is obtained automatically if 





Fig. 6. Curve showing change of refractive index {n) 

 with distance {x) in the cell, i.e., dn/dx, plotted against 

 position along the cell, x. 



the photographic plate of the camera is moved uniformly across the 

 cell image as the schlieren diaphragm is raised. This scanning 

 method for obtaining dn/dx versus x patterns was introduced by 

 Longsworth. Such curves describe the total refractive index change 

 across the boundary and thus the total concentration change in pro- 

 tein across the boundary. The area under the curve is, therefore, 

 proportional to the protein concentration change at the boundary. 

 In the region of the boundary, dn/dx increases to a maximum (at the 

 center of the boundary, if symmetrical) and decreases to zero in the 

 region away from the boundary. If more than one electrophoreti- 

 cally different protein is present, a scanning picture taken after the 



