828 Reflexion of X-Rays from Imperfect Crystals. 



interpretation of experiments will be freed from a source of 

 error, the amount of which must be very uncertain. 



We therefore conclude that, to establish beyond doubt the 

 validity of the work of Bragg, James, and Bosanquet, it is 

 essential that the work of § § 7, 8 should be verified by tests 

 like those of § 9, and if it should be proved correct, then we 

 may have great confidence in their results. Failing this 

 verification, the theory on which they eliminated the extinc- 

 tion is without good foundation and the results must be 

 regarded with some caution. In this case, it seems to me 

 that the most satisfactory way of determining Q is by the 

 powder method of § 10. 



Summary. 



The paper is a theoretical inquiry into the possibility of 

 determining the arrangement of electrons in the atom from 

 the intensities of the X-rays scattered by crystals. This 

 problem falls into two stages : first from crystal to molecule, 

 then from molecule to electrons — only the first stage is here 

 treated. 



Simple formulae have been given by various writers, and 

 the process has been carried out experimentally by Bragg, 

 James and Bosanquet. They encountered the difficulty of 

 "extinction." This extra absorption falsifies the formulae, 

 but they measured it directly and so obtained a correction. 

 This paper is concerned with seeing whether their correction 

 was valid. The point of the problem was known to lie in 

 the imperfection of crystals. 



After a general discussion (§§1,2, 3), it is shown (§4) 

 that if a small perfect crystal of any shape is turned through 

 the reflecting angle for monochromatic rays, the amount of 

 reflexion determines a quantity Q, which is what is required 

 for the second stage of the problem. 



The reflexion is worked out (§ 5) for a conglomerate of 

 small blocks of perfect crystal all orientated nearly in the 

 same direction, the conglomerate being so thin that absorp- 

 tion and extinction can be neglected. 



Extinction — that is, the special absorption of rays at the 

 reflecting angle — is shown (§ 6) to lead to two effects, 

 primary and secondary. The primary diminishes the 

 reflexion from a perfect crystal below the amount given 

 by the simpler theory. It leads to a change in the value 

 of Q depending on the depth of the crystal, and none of 

 the experimental processes eliminate this change. The 

 secondary extinction results from the reduction in the 

 strength of the beam transmitted through the crystal. 



