The Intensity of Reflexion of X-Rays by Rock-Salt. 9 



To quote from Darwin's paper : ie . . . We have strong 

 experimental reason to believe that the crystals are even 

 more imperfect than this. For when the reflexion is 

 evaluated ... it will be found that the second order of 

 reflexion is as strong as the first — a result known to be 

 untrue. This must be taken to indicate that the crystals 

 are so badly twisted that their planes do not remain parallel 

 even long enough to produce a single perfect reflexion/' 



We may therefore distinguish two degrees of irregularity 

 in a crystal structure. On the one hand, the crystal may 

 be so irregular that the "absorption in each homogeneous 

 fragment is entirely due to the conversion of the X-ray 

 energy into cathode-ray energy. The absorption-coefficient 

 will then be the same for rays passing through the crystal 

 in any direction. This absorption-coefficient, which we 

 will call yL6 , can be found by direct measurement, and, 

 in the case of these very irregular crystals, it is the correct 

 coefficient to substitute in all the formulae for reflexion. 

 On the other hand, the homogeneous fragments, while still 

 irregularly arranged, may each be sufficiently large for 

 the extinction of the transmitted beam at the reflecting 

 angle to be appreciable, in comparison with the reduction 

 in intensity due to normal absorption. In this case, 

 X-rays, passing through the composite crystal at such an 

 angle that some of the fragments are reflecting, will be 

 absorbed more strongly than rays passing through at 

 other angles. There will be an increase in the effective 

 absorption-coefficient. If the effective coefficient of ab- 

 sorption is now //, we can state that 



The coefficient e will be called the "coefficient of extinction" 

 of the X-rays in the crystal. 



The Measurement of the Extinction- Coefficient. 



6. Our experimental results show that the regularity in 

 structure of rock-salt is such that e is appreciable for strong 

 reflexions, but is very small for the reflexions of high order. 

 We have measured the normal coefficient of absorption fi 

 for rays passing through the crvstal at any angle, and 

 found it to be equal to 10*7. We have then determined 

 the effective coefficient of absorption fi at the reflecting 

 angle by the method described in paragraph I. This has 

 been done for the reflexions (100), (110), (200), and (300). 

 The results are shown in figs. 4, 5, and 6. 



