backward, the beam is converged on the specimen by the mirror block, 

 and is diffracted to the photographic phite where it registers a pattern 

 characteristic of the specimen. 



In X-ray diffraction, a narrow beam of X rays is passed through the 

 object, some of the rays being scattered as the result of diffraction (or 





I « 



♦ ♦ ♦ « 





Figure 11-35. X-Ray Diffraction Pattern of Micro-crystalline Fibers of 

 Lithium Salt of DNA in the /j configuration. (Courtesy of Dr. M. H. F. 

 Wilkins, Biophysics Research Unit, King's College, London.) 



reflection) by the atoms of the object. The diffracted rays on emerging 

 from the object travel in various directions depending on the angle at 

 which they are diffracted. This pattern of diffraction is recorded by 

 means of a photographic plate placed a short distance (5 to 10 mm) 

 beyond the object. In the case of crystalline materials, the lattice struc- 

 ture of the component crystals tends to concentrate the diffracted X rays 

 in certain definite directions to produce a characteristic diffraction pat- 



256 / CHAPTER 11 



