﻿524 Mr. S. B. McLaren on Emission and Absorption 

 as in (44), 



3 



since <£ is zero at S. Hence the above equation reduces to 



K> + Kj)\F^~dD = 4*y£F m di 



Multiply (28) throughout by ¥ m dv and integrate, using 

 (44) and (45). Then 



h>=§&J?»do (46) 



d 2 u m , 1/1 ^ 2 <£.„\ J7 s 



Write 



Then 



1 o 

 c 



And in (39) 



3> = 2 «»&, (48) 



The equations (41) and (47) represent completely ihe 

 interactions of the field of radiation and the material system. 



§5. The Perturbed Motion. 



The course followed by the material system is slightly 

 disturbed by the field of radiation represented by the terms 

 in <t> in (41). Hence there is absorption of the radiant 

 energy. The deviation produced in the immediate past 

 alone causes the absorption at any instant. In the simple 

 theory of metallic conduction it is the deviation arising 

 within the time of a free path. The collision which 

 preceded that free path has wiped out all that went before. 

 Whatever the character of the dynamical system it is evident 

 that the intramolecular forces determine the path it follows, 

 and that the deviation which is connected with the absorption 

 at time t is that which has been produced within some very 

 small interval of time e preceding. 



Let c l c 2 ...c<,n be 2n constants representing coordinates 

 and momenta at any time in the undisturbed motion. Let 

 Bp r and Sq r be the deviations in the values of p r and q r at 



