602-605] 



Metallic Media 



525 



by equations (581) and (582). For light of sufficiently long wave-length, this 

 relation ought to give the value of the optical quantity 100 - R in terms of 

 the wave-length and the electrical constants of the metals. 



An extremely important series of experiments have been conducted by 

 Hagen and Rubens* to test the truth of the formula for light of great wave- 

 length. The following table will illustrate the results obtained! : 



In the calculated values, the value of K is assumed to be unity, and an 

 error is of course introduced from the fact that the wave-length dealt with, 

 \ = 12/j,, although large is still finite. The authors say: "Excepting the 

 values given for bismuth, the agreement is good, particularly when we con- 

 sider that the numbers calculated from formula [583] are absolute values and 

 do not contain any arbitrary coefficient." A series of experiments with light 

 of the still longer wave-length A.= 25'5/A shewed an even better agreement 

 between theory and observation. 



605. When we pass to experiments with light in which the wave-leugth 

 cannot be treated as large, the values given by theory for the various optical 

 quantities are not in good agreement with those observed experimentally. 

 Drude has conducted a series of determinations of the optical constants of 

 metals, which shews very clearly the discrepancy between experiment and 

 the theory which we have given. The requisite modification of the theory 

 is also due to Drude. Our theory has treated the metallic medium as 

 uncharged, and so has taken no account of the presence of moving electrons 

 inside the conductor. As soon as the motion of these electrons is taken 

 into account, theory arid experiment are reconciled, not only with respect to 

 this discrepancy, but with respect also to that mentioned in 601. In the 

 following sections ( 606 609) we shall give a brief statement of the 



* Annalen der Physik, 11, p. 873 ; Phil. Mag. 7, p. 157. 

 t Phil. Mag. 7, p. 168. 



