of Rays in X-ray Diffraction. 



303 



This would seem to suggest that possibly the mechanism 

 of scattering is not so simple as supposed but involves a 

 more indirect process such as absorption and subsequent 

 re-emission of energy. Because of its very great theoretical 

 importance it has seemed worth while to test this matter 

 with considerable care. The best known method for ob- 

 taining monochromatic rays is by crystal reflexion, but the 

 energy of a reflected beam is of the order of one one- 

 thousandth of that in the incident beam within the same 

 frequency limits, and it has hitherto been thought imprac- 

 ticable to attempt accurate measurements of the scattered 

 tertiary radiation produced by these weak reflected beams. 

 One of the chief difficulties is in shielding the ionization 

 chamber from all extraneous scattered radiation such as that 

 from the slits bounding the primary and reflected beams. 



The experimental arrangements finally adopted are shown 

 schematically in the figure. A powerful radiator-type 



Coolidge tube (1) was used with a target which gave a 

 strong isolated line at moderate voltages, such as rhodium 

 or molybdenum. The reflector was mica, bent into an 

 equiangular spiral. The scattering substance (3) was 

 usually paraffin or water. An ionization chamber (4) 

 rotates about a centre at (3), and (3) with (4) rotates 

 about a centre at (2). A lead screen (5) catches the 

 primary beam, and lead screens (7), (8), (9) restrict the 



