due to the Scattering of Light by Electron*. 717 



If the atoms are orientated uniformly, the mean value of 

 this expression is 



Z./ i i + ..J \ 



•J mXnf—p? n 2 2 —p 2 >h 2 — p 2 J 



So that if a is the density of the electrons, the value of /jl 

 the refractive index will be 



1 + ** t± (—1— + _L_ + 1 \ / 5 } 



3 w Ui 3 -/ no 2 — p 2 n 3 2 —p 2 )' 



Thus if w 2 and ?> 3 were very large compared with n u 



2tt e 2 a 1_ 



/a_1+ 3 m nf-p 2 ' 



thus the true value of a would be three times that obtained 

 by the usualformula 



m n* — jtr 



The usual formula 'is only true when the displacement of 

 the electron is always in the direction of the resultant force; 

 when this is so n 1 =zn 2 — n. 6 , and the expression for /j, takes 

 the usual form. 



Conduction in Metals. 

 If we suppose that the velocity of the electrons is pro- 

 portional to the electric force, then 



dz = Vf 



dt ea ' 

 where C is the specific conductivity of the metal ; for 



ea' j is the current through unit area. Using this value of 

 at 



d: dt equation (2) gives 



df 1 df 2ttC 



(Lc 

 The solution of which is 



dx + c dt c *' 



2*0 



( = € . </)(.!' — Ct). 



Since the intensity of the radiation is proportional to/ 2 , 

 the rate of decay of this intensity is 



tC 



5 



so that the coefficient of absorption is equal to Att( ! <•. 



