256 



PROF. W. H. BRAGG ON X-RAYS AND CRYSTAL STRUCTURE. 



In practice a fine pencil of X-rays is allowed to fall upon a crystal face, natural or 

 prepared, and the crystal is gradually turned so that the angle between rays and 

 face, called the glancing angle, steadily increases from zero onwards. Reflection 

 takes place whenever the formula is satisfied, the reflected pencil being detected by 

 the ionisation which it produces in a chamber containing a heavy gas. The chamber 

 can be made to turn about the same axis as the crystal. 



If the incident rays are heterogeneous and contain pencils of various wave-lengths, 

 each is reflected at the proper angle, and the rays are analysed, forming a spectrum. 

 The figure shows the analysis of the rays issuing from an X-ray bulb in which the 



4-0' || 20' 4-0 ' (2 20' <K>' 



Fig. 4. Spectrum of Rh X-rays. Each dot records a separate measurement. 



anti-cathode is made of rhodium ; the spectrum is therefore characteristic of rhodium. 

 The abscissa are the various values of the glancing angle, the ordinates are the 

 measured intensities of the reflected rays. It may be mentioned that glancing angles 

 can be measured with little difficulty to a minute of arc. Intensities can be measured 

 to one or two parts in a hundred if the circumstances are not varied. If the experi- 

 ment is repeated under fresh circumstances the agreement is not quite so good. 



It will be observed that the rhodium spectrum contains four " lines." In other 

 words, when stimulated by the impact of the cathode stream rhodium emits four 

 pencils of different wave-lengths. These have been calculated* to be 0'534, 0'545, 

 0'614 and 0'619 A.U. respectively. The crystal planes here used are the ( 1 1 1 ) planes 

 of calcite, those which are perpendicular to its axis. The spacing is 2'83 A.U. The 

 doublet a u a 2 is very convenient for crystallographic work. 



If we always make use of the same homogeneous X-rays, for example of the line 2 

 (or, which is much the same thing in most experiments, the doublet a 1; a 2 ) we can 

 compare the spacings of different sets of planes, whether of the same or of different 

 crystals. This is one step towards the determination of crystalline structure. In 

 order to make its significance clear it will be well to consider certain elementary 

 principles of crystallography. 



However complicated may be the arrangement of the atoms in a crystal with 

 respect to each other they must allow of being grouped into units which are repeated 

 regularly throughout the crystal. Every unit resembles every other exactly as 

 regards both the internal disposition of its members and the external disposition of 

 group to group. The grouping may, however, be effected in more than one way. If 



* 'Phil. Mag.,' March, 1914. 



