78 



E. G. PICKELS 



light, usually ultraviolet, and of such wavelength as to be differen- 

 tially absorbed by solute and solvent, is directed through the revolving 

 cell. Since the sedimentation occurs radially, the meniscus of the 

 fluid in the cell would appear to the eye as a fine line curved about the 

 axis of rotation. The sedimenting boundary is also concentric about 

 the axis of rotation. A record of the sedimentation is obtained by pho- 

 tographing at intervals only a narrow cross-sectional strip of the total 

 annular band (1). A camera lens of long focal length (100 cm.) is 

 used to avoid errors of parallax and to give good depth of focus. Typi- 

 cal photographic records are shown in Figures 4 and 5. 



A 



67,5 MM. TO AXIS 

 OF ROTATION 



MENISCUS 

 or FLUID 



^'r 



15 



MM. 



2k 



■■'■■III 



K^ 



he 



LIGHT INTENSITY REFERENCES, 



Fig. 4. Sedimentation of a monodisperse, high molecular protein (hemo- 

 cyanin, approximate molecular weight 3.5 X 10^) as recorded by the ab- 

 sorption method. Photographs taken fifteen minutes apart with ultra- 

 violet light; speed, 18,000 r.p.m. 



The original absorption method has been largely superseded by 

 methods {4,2 1-24) based on the detection or measurements of devia- 

 tions suffered by light rays passing through regions of refractive index 

 gradient, such as exists at sedimenting boundaries. The two principal 

 methods in current use are the Lamm scale method {24,26) and the 

 cylindrical lens method {4,23). In the first of these a real or opti- 

 cally projected scale is placed between the revolving cell and a dif- 

 fuse source of illumination. The equally spaced (not more than a few 

 tenths of a millimeter) lines of the scale are oriented tangentially with 



