36 



DISCOVERY 



constitutes the crystal, they are merely the evidence 

 of the internal regular arrangement. 



The exact way in which the molecules of a crystal 

 are arranged has always been a matter of great interest, 

 but the extreme minuteness of the pattern has imtil 

 recently been an insuperable obstacle to investigation. 

 When examining small objects with a microscope the 

 power of resolution is limited, not by the impossibility 

 of designing a better instrument, but by the size of 

 the light waves which we use to illuminate the object. 

 It is impossible to see any detail of a body smaller 

 than the wave-length of hght, because the wave- 

 trains coming from the different parts of the body 

 cannot be distinguished from each other. The finest 

 detail we can hope to examine with a microscope 

 corresponds to the shortest wave-length of hght we 



the atoms in a crystal, and that "they provide a source 

 of illumination which is capable of reveaUng its detailed 

 structure. In carrying out his experiment, Laue 

 expected to get an effect on the X-rays analogous to 

 that of a diffraction-grating on light. The diffraction- 

 grating consists of a plate of polished metal on which a 

 very large number of fme lines have been ruled by a 

 diamond point at regular intervals, spaced so that there 

 are from ten to twenty thousand lines to the inch. 

 When hght falls on such a grating, each line scatters or 

 " diffracts " a small portion of the incident light. 

 If hght of definite wave-length is examined, there are 

 certain directions in which the wave-trains scattered 

 by the hnes of the grating are exactly in phase with 

 each other, the crests and troughs of the waves coincid- 

 ing in position, so that they combine to form a strong 



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NICKEI. SUI.PHATE. 



can use. Taking as unit of length the Angstrom unit, 

 which is one hundred-millionth of a centimetre, we 

 cannot see detail finer than about 3,000 Angstrom 

 units. The distances separating the molecules in a 

 crystal we know to be of the order of two or three 

 Angstrom units, cind so they are far beyond the 

 possibihty of microscopic investigation. 



The discovery which opened the way towards 

 finding how a crystal was built was made in 1912 by 

 Laue. He was inspired to try the effect of passing a 

 beam of X-rays through a crystal. It had already 

 been suspected that X-rays consisted of electromagnetic 

 waves of exactly the same nature as heat and light 

 radiations, but with a wave-length which was one 

 ten-thousandth that of visible light. Laue reaUsed 

 that such waves are smaller than the distances between 



diffracted beam. In other directions the waves are 

 not in phase, and the general effect is zero. The 

 analysis of a spectrum by a grating depends on this 

 principle. Laue reahsed that a crystal provides a 

 diffraction grating where the regular arrangement of 

 molecules takes the place of the hnes ruled on a metal 

 grating, and where the distance between the molecules 

 is of the right dimensions to diffract the very short 

 X-rays. 



The results which he obtained when he passed a 

 beam of X-rays through a crystal are shown in Fig. 2. 

 In his apparatus a very narrow pencil of rays fell on 

 a small slip of crystal, and a photographic plate was 

 placed on the far side at right-angles to the rays. The 

 large spot in the centre of the plate is produced by 

 the direct beam of rays, the pattern of small spots 



