416 



Dr. L. Vegard on 



In the way described we easily find the following ex- 

 pression for the amplitudes : 



(100) A n = I + N + 2C(1 + cos 2na') + 2H [1 + 2 cos 2nfu! 



+ 2 cos 2na'(l +/) + cos 2n«'(l + 2/)], 



(110) A n = (I + N)(-l)" + 4Ccosna' 



-f- 4H [cos W + 2 cos na' (1 + 2/) ] , 



(001) A n =(N + 4C+(-l) w l)cos/i^ 



+ 4H [cos n |- + 2 cos n |- (1 + 2/)"| , 

 (101) A n = [(I + N + 2G)(--l)» + 20cosn«']cosn(|-^) 



+ 2H(-l)^[cosn| + cosn(|-/(y-a')) 



+ cosn(|-/( 7 ' r *'))] 

 + 2Hrcos^Y^ + a'-/(7 , -a / )) 



+ cosn^-a'-y( 7 / 4a / ))] 

 + 5[cosn0 + a' + 2/* , y+cosn(|-V-2/a')], 



(lll)|'A n =(l4-(-l) n N + 4CcosW)cosn| 



+ 2H|cosn[(|-a / Vl + 2/)]+cosn^-a'+/ 7 '') 



+ coS7 i ('|--*'-2/a') + cosn('|-+a' + 2/aM 



+ cos(|' + a^/y) + cosn[(|-' + ^)(l + 2/)]}. 



Also these formulae, which take into account the effect of 

 the hydrogen atoms, will give values of 7' and a' which 

 very nearly satisfy the condition (15). The value of a! is 

 most easily determined from the (110) face and that of 7' 

 from the (001) face ; since for the first face the amplitudes 

 only depend on a and /, and for the second one only on 7' 

 and f. 



The amplitudes of these faces are also very sensitive to 



►.(18 a) 



