82 



E. G. PICKELS 



position corresponding to B. The corresponding point B' will be the one 

 suffering the maximum horizontal displacement from the center line on the 

 screen. Light coming through the cell below and above the boundar}^ as at 

 A, is not deviated and the corresponding light points fall on the center line. 



Displacements of light points corresponding to successive positions 

 within the boundary are proportional to the respective refractive index 

 gradients, mth the result that the light points form a smooth continu- 

 ous curve on the screen. With the ultracentrifuge there are extrane- 

 ous gradients due to distortions of the cell, hydrostatic compression 

 within the contained fluid, and the partial sedimentation of salts or 

 other materials incorporated in the suspending medium. The height 

 and shape of the "base Hne" curve (with reference to the undeviated 

 light through the counterbalance) vary slightly during the course of 



B 



Fig. 7. Refractive index photographs showing single boundary (A) in a rela- 

 tively monodisperse preparation of 0.8% hemocyanin, and multiple boundaries 

 (B) produced by irradiating the same material with X rays. 



a centrifugation because of the continued redistribution of the sedi- 

 mentable material within the medium. Hence, it is necessary to ob- 

 tain a series of base line photographs during a duplicate run with the 

 suspending medium alone. The difference (with respect to the un- 

 deviated light) between the curves obtained after comparable centri- 

 fugation times with and without the principal sedimenting solute 

 represents the effect due to this solute. The differential curve will 

 be nearly symmetrical if the material is monodisperse and the peak 

 will represent the position of the boundary. There will be a peak 

 or hump for each differentiated sedimenting component. The area 

 beneath each peak is proportional to the concentration of the respec- 

 tive component. 



