CONTEMPORARY ADVANCES IN PHYSICS 409 



only half-a-dozen rulings would cast a spectrum of indistinct wide 

 fringes instead of fine sharp "lines." It takes a multitude of atom- 

 groups or rulings to produce the efficient destructive interference 

 which etches out the borders of these blotches and leaves the centres 

 standing up in high relief, — which in technical language makes the 

 resolving-power high. In actual crystals, are there atom-groups 

 enough ? 



Fig. 11 — Laue pattern of an iron crystal, showing that these crystals are built 

 on a cubic lattice, though they are seldom so shaped as to reveal the fact. (G. L. 

 Clark.) 



In actual crystals, there are usually plenty. Infinite resolving- 

 power or infinite sharpness of diffraction-pattern would of course 

 imply infinitely many groups arrayed at perfectly regular intervals; 

 but infinity and perfection are not workable ideas in physics. The 

 practical question is, whether the actual defects of the diffraction- 

 pattern from infinite sharpness arise because the crystal lattices are 

 not prolonged enough, or from other causes. Usually they are due 

 to other causes, which are numerous; for instance, appreciable breadth 

 of the primary beam and appreciable size of the crystal. In the rare 

 cases where the fuzziness of the diffraction-spots betokens that the 

 crystal lattice is limited, the fact is often of scientific importance. 

 The size of exceedingly small — submicroscopic — crystals can be 

 determined in this way; the dimensions of colloid particles, and of 

 the crystals in metals, are estimated thus. 



(ii) The assumption of stationary atoms. This of course is faulty, 



for the atoms are in thermal agitation. Their oscillations make the 



spots of the diffraction-pattern somewhat hazy, and alter furthermore 



their relative intensities. Out of this circumstance the physicists 



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