( 678 ) 



2ürc 

 This is found from (25) by using the relation n =z ---. 



A 



§ li. The ]-esuh of the preceding ealcuhxtions not oidy conforms 

 to the law of Kirchhoff ; it lias also a form agreeing with those of 

 BoLTZMAXN and Wien. Ijideed, the expression (26) follows from (1), 

 if we put 



16 



Our last task will he to evaluate the constant « by ap[)lying the 



formula (26) to experimental determinations of the radiation of black 



bodies, and to compare the result with what has been ijiferred about 



the same constant from other classes of phenomena. Combining the 



measurements of Lummer and Pringsueim *), who have gone tar into 



the infra-red, with the aI)solute amount of the radiation as determined 



by KuRLBAL'M ^), 1 find 



era 

 «=1.G . 10-16 ^£_. 

 degree 



On the other hand, we get, starting from van der Waals' evalua- 

 tion of the mass of an atom of hy'drogen, 



«= 1,2 . 10-10. 



A comparison of my formula with that of Planck is also interesting. 

 For very large values of the product XT, the dcuoininator in 



(2) becomes , aiul the expression itself — -— d?.. This agrees with 



3 , 

 (26), if a=i -~k. 



Now the mean kinetic energy of a molecule of a gas would be 

 — /-■ 7' according lo Planck and has been represented in what pre- 

 cedes by « T. There appears therefore to l)e a full agreement bet ween 

 the two theories in the case of long waves, certainly a i-emarkable 

 conclusion, as the fundamental assumptions are widely diilerent. 



On the absorption hy a thin nietaUic plate. 



§ 12. Take the origin of coordinates in the front surface, the 



axis of z towards the metal, and let there be free aether on both sides. 



Writing (£• for the electric force, j for the current of conduction. 



1) Lummer and Pringshetm, Verhandl. d. deutschen phys. Gesellsch., I'JOO, p. 163. 



2) KuRLBAUM, Wied. Ann., Bd. 65, p. 754, 1898. 



