M. F. PERUTZ 





+Z 



Figure 4. Contour map 

 obtained by adding den- 

 sities of sections 6*3 #/:</ 

 7'4 A cfove f/?e origin. 



y = 9/60 + 10/60 + 11/60 

 .X 



Figure 5. Contour map 

 obtained by adding den- 

 sities of sections 9' 5, 10" 5 

 and 11*6 A above the 

 origin. 

 {Reproduced from II.) 



parallel to the A'-direction which are shown in Figures 4 and 5. These 

 contour maps were drawn by superimposing two (or three) sections 

 and adding the densities at each point. In this way features which 

 extend over more than one section can be made to show up very 

 clearly. In Figure 4 two rods of high vector density (marked A-A) 

 are visible ; these are really part of one continuous rod-like structure 

 which runs through most of the unit cell — a fact which can be visualized 

 if the map is imagined to be repeated by symmetry, involving rotation 

 through 180° around the points x = z = and x = J, z = 0. 

 Finally Figure 5 shows a structure, rod-like though rather tortuous, 

 winding its way along the A'-direction at the centre of the diagram 

 (marked B-B). Both the 'A' and the ' B' rods are spaced 10-11 

 A from the central one. As they are all parallel to the ^-direction 



140 



