IX. ELECTROPHORESIS 



293 



passes through such a region of changing refractive index, its path 

 will be bent toward the region of higher refractive index. The index 

 of refraction of the protein solution will be greater than that of the 

 overlying solution by an amount ])r()poi'lional to the concentration of 

 the protein and to its refractive iiicremenl. 



Figure 5 shows diagrammatically the manner in which this prin- 

 ciple may be applied to the detection of the i)()sition of a boundary in 

 the Tisehus cell. A horizontal slit, source of light (S) is allowed to 

 pass through a long focal length (schlieren) lens (L), which forms an 

 image of the slit at point P. If the light in passing from the lens to 

 point P encounters no region of refractive index change (existing in a 

 direction perpendicular to its direction of propagation) all the light 

 will be brought to focus as a simple image of the slit at P. If, how- 



Fig. 5. Schlieren method for detecting position of a boundar}^ (refractive i 

 dex gradient) in the macroelectroplioresis cell. 



m 



ever, an electrophoresis cell (E) is placed in the path of the beam from 

 L to P and this cell contains a boundary at X across which there is a 

 refractive index change in a vertical direction, that fraction of the 

 light passing through this region of refractive index change will be 

 bent down and will be brought to focus in the same plane as P but at 

 a point vertically below P. If a camera of long focal length is focused 

 on the electrophoresis cell with the camera lens (C) placed just beyond 

 the point P in the path of the light beam, all the light passing the cell 

 will be brought to focus on the camera plate. When a diaphragm 

 (D) is raised in the plane of P to a point where it will intercept the 

 light thrown down but not the main beam, there will appear on the 

 camera plate a shadow (X') in the image of the cell (E') corresponding 

 to that region containing the refractive index gradient, i.e., the region 

 of the boundary. Thus the position of the boundary in the cell can 

 be determined from the position of the shadow in its image. (See 

 also Figure 6 of Chapter III.) 



The degree to which the light beam passing through the boundary 



