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SCIENCE PROGRESS 



differed by £ per cent. But the accuracy with which the wave- 

 lengths of separate lines can be compared is probably very 



X 



Z2 



Fig. i. 



(a) Shows what would happen if there were an insufficient numher of reflecting planes in the crystal 

 or an insufficient number of waves in the X-ray train. A fine pencil of X-rays limited by slits in blocks of 

 lead would, after striking the crystal, be opened out into a diffused and spreading pencil. 



(i) Shows what would happen if the crystal was badly built : for example, if there were two portions 

 of crystal, not quite in alignment with each other where the X-rays fell upon them, the original ray might 

 be broken into two. 



(c) Shows how, even if everything was perfect and the X-rays were reflected very close to the surface 

 of the crystal the reflected beam must be as wide as the original beam. 



(d) Shows how the reflected beam must be wider than the incident beam on account of the fact that the 

 reflection is from the body of the crystal and not only from the surface. 



much higher than such a figure would seem to imply, and this 

 is true even of the absolute measurements themselves. For 



