884 



Prof. W. H. Bragg on the Intensity of 



not, however, give us a proper measure of • the intensity of 

 reflexion. For, whereas the current in the latter position is 

 proportional to the width of the slit at B, the reflexion in the 

 former case is not proportional to the width of the same slit. 



Reflexion takes place only when the angle between ray and 

 crystal is very exactly adjusted. If we consider the rays 

 which come from a single point in the source at A and 

 fall upon a perfect piece of crystal, a very narrow slit at B 

 will let through all the rays which can be reflected at one 

 time, and the reflexion is not increased by opening the slit 

 any wider. The comparison of quantities will therefore 

 bring in the width of the slit, and it cannot be eliminated 

 without a better knowledge of theory than we possess. 



But if we turn the crystal by a succession of small steps 

 through the angle at which reflexion takes place, measuring 

 the ionization current at each step, and if we plot the results 

 in a curve, the area of that curve is a measure of the reflexion 

 effect which does contain the width of the slit as a factor. 

 I£ we open the slit at B we prolong in proportion the range 



