﻿446 Prof. W. L. Bragg and Messrs. James and Bosanquet : 

 This gives the following figure for F^ a : — 



Sin 9. 0-1. 0-2. 0-3. 0-4. 0'5. 



F calculated 

 F observed . . 



•73 



5-04 



376 



.2-53 



1-80 



•32 



5-40 



3-37 



2-02 



0-76 



The agreement, of course, is not perfect, but one must 

 remember that no attempt has been made to adjust the size 

 ■of the orbits to fit the curve. The method of calculation 

 too is very rough, although it must give results of the right 

 order. The point to be noticed is that the curve is quite of 

 the right type, and there is no doubt that an average distri- 

 bution of electrons of the nature given by such an atom 

 model could be made to fit the observed value of F quite 

 satisfactorily. 



7. The points which appear to us to be most doubtful in 

 the above analysis of our results are the following : — 



(a) We have assumed that each electron scatters inde- 

 pendently, and that the amount of scattered radiation is that 

 calculated for a free electron in space according to the 

 classical electromagnetic theory. It is known that for very 

 short waves this cannot be so, since the absorption of 7 rays 

 by matter is much smaller than scattering would account 

 for, if it took place according to this law. On the other 

 hand, the evidence points towards the truth of the classical 

 formula in the region of wave-lengths we have used 

 (0-615 A). 



(b) We have used certain formulae (given in our previous 

 papers, to which reference has been made) in order to 

 calculate the quantity we have called Q in equation (2) 

 from the observed intensity of reflexion of a large crystal. 

 Darwin * has recently discussed the validity of these 

 formulae. The difficulty lies entirely in the allowance which 

 has to be made for "extinction" in the crystal. X-rays 

 passing through at the angle for reflexion suffer an increased 

 absorption owing to loss of energy by reflexion. 



Darwin has shown that this extinction is of two kinds, 

 which he has called primary and secondary. If the crys- 

 talline mass is camposed of a nuaiber of nearly- parallel 

 homogeneous crystals, each so small that absorption in it 

 is inappreciable even at the reflecting angle, then secondary 

 extinction alone takes place. At the reflecting angle the 



* Phil. Mag. vol! xliii. p. 800, May 1922. 



