250 



PROTOPLASM 



cellulose is symmetrical, that is to say, crystalline. Spectro- 

 grams are pictures of diffraction phenomena resulting when 

 X rays are broken up by the lattice-like structure of crystals. 



X rays were long thought to be 

 similar to Ught rays, except 

 that they did not show diffrac- 

 tion. This was due to their 

 very short length. To scatter 

 any type of wave, one must 

 have an obstacle as small as 

 the wave. Diffraction gratings 

 of 1,000 lines to a millimeter 

 (the lines would then be 1 /z 

 apart) will break up ordinary 

 light waves (of 0.5 m) but not 

 such minute wave fronts as 

 those of X rays. The German 

 physicist Laue had the brilliant 

 idea that the latticework of 

 atoms which constitutes the structure of crystals (Fig. 143) 

 presents a grating sufficiently fine to disturb X rays. His 

 prediction was verified, and the discovery now serves as a 



Fig. 129. — X ray spectrogram of cellu- 

 lose. {From A. W. Kenney and H 

 Aughey.) 



Fig. 130. 



-The elementary cell of cellulose. {From K. H. Meyer.) 



means for establishing the crystalline nature of substances 

 on the basis of their X ray diffraction patterns. Not only 

 does a symmetrical distribution of bright points on the 



