801 Mr. C. G-. Darwin on the Reflexion of 



seen, this complicates the mathematics of the problem 

 considerably. 



With regard to the extinction itself it is found that it exerts 

 two effects, which may be called primary and secondary. 

 The primary extinction consists in the reduction of the beam 

 reflected from a perfect crystal, owing to the defect in the 

 radiation which reaches its lower layers. As shown in D. ii. 

 it may be deduced by considering the multiple reflexions 

 between the planes of' the crystal, and for a deep crystal it 

 leads to perfect reflexion in a region near the reflecting 

 angle. At first sight this is a little paradoxical, but it is 

 easy to see that the simpler formula, which neglects extinc- 

 tion, gives an amount reflected greater than the amount of 

 the incident beam. 



The secondary extinction is due to the reduction in in- 

 tensity of the transmitted beam on emerging from the lower 

 side of a small crystal in which some reflexion has taken 

 place. Its effect is practically to increase the absorption 

 coefficient of the crystal by an amount that can be calculated 

 from the amount of the reflexion. The methods used in 

 B.J.B.ii. remove the secondary extinction, but are without 

 influence on the primary. In fact, it will appear that no 

 experiments of the present type can possibly remove it; 

 indeed, to do so would require the measurement of the 

 actual sizes of the small blocks of perfect crystal. This is 

 a serious difficulty in the problem of determining with cer- 

 tainty the positions of the electrons in the atom ; but it 

 should be said that it seems probable that in rock-salt the 

 secondary extinction is far more important than the 

 primary: for, if the imperfection is due to warping rather 

 than cracking, there will be very little primary extinction 

 (which depends on the depths of the perfect crystals), 

 whereas the secondary extinction will be as effective as 

 ever. 



A confusing circumstance of the problem has lain in the 

 different physical dimensions of the qmmtities that occur. 

 For example, it is natural to measure the incident beam by 

 its intensity, erg.cm." 2 sec. _1 , whereas the whole reflexion is 

 required, and this is of dimensions erg. sec." 1 . This type ot: 

 difficulty is illustrated in Bragg's formulae, which involve a 

 superficially irrelevant angular velocity. I have found it 

 very helpful to adopt the terminology of dimensions for the 

 various quantities occurring. Thus we shall denote the whole 

 ionization produced in the electroscope in a given time as 

 energy. Then, following the dynamical usage, power will 

 signify energy per time. Intensity will be power per area, 



