80 



E. G. PICKELS 



tion of the system used on the vacuum ultracentrifuge already de- 

 scribed and of the optical principles involved is given in Figure 6. 

 Consider the first arrangement shown, with the thin object diaphragm 

 placed between the collimating and condensing lenses. All elements 

 are centered on an optical axis and disposed along it to fulfill the fol- 

 lowing conditions : light rays from a common point in the slit source 



Object plane 



Image of source 



Screen 



Horizontal 

 slit source 



Diaphragm with 

 vertical slit 



Fig. 6. Perspective schematic drawing illustrating working principle of cylin- 

 drical lens method. 



are parallel after passing through the collimating lens; the condens- 

 ing lens forms an image of the slit source in front of the camera lens 

 as shown; sharp images (A', B', C) of the small holes {A, B, C) in 

 the object diaphragm are formed on the screen by the camera lens 

 and the condensing lens when the diaphragm is diffusely illuminated. 



Limit consideration to those rays that pass through the small holes. 

 Each set of rays corresponding to one of the holes may be thought of as 

 making up a "light sheet" that varies in width as it progresses but is always 

 parallel to the horizontal slit source. If the light sheet diverging from A to- 

 ward the condensing lens were deviated downward (by a prism, for example) 

 from the normal course at A, as indicated by the broken lines, it would 



