264 



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



A fine pencil of X-rays falls upon a crystal, and the ionisation chamber is set to 

 receive the reflection. The crystal is first set at an angle which is just so far from 

 being correct that no reflection takes place. It is then made to revolve steadily, or 

 rather by uniform small movements at regular intervals ; it passes through all the 

 positions in which it can reflect and when the angle has again become such that no 

 reflection takes place, the total movement of the electroscope leaf is observed. In 

 this way the whole reflection effect is integrated. This method is fully discussed in 

 the paper quoted. It is enough to say here that such measurements are really 



-AU. 



10 



Fig. 14. 



comparable with each other, and in particular that the effect of crystal imperfections 

 (sometimes serious) are avoided. A small allowance has to be made for the reflection 

 and scattering of general radiation which enters the slit with the reflected pencil 

 actually measured. This is easily done. 



Let us now consider the interpretation of these measurements of intensity. Very 

 useful information on this point is to be gathered from a study of the spectra of various 

 calcite planes. The positions and intensities of the spectra of six of the most 

 important planes are shown in fig. 14. The diagram also shows the spacings of 



